Baptism was practiced before the time of John the Baptist. In fact, Adam, the first man, was baptized as was Eve, Seth, and many of Adam's posterity. The origin of the word "baptism" is Greek and literally means to dip or immerse.

The Law of Moses refers many times to washings, however most of these references are not in relation to the ordinance of baptism in question. John the Baptist was of Priestly Descent which gave him authority to administer the ordinances associated with the Law of Moses but he was also ordained by God prior to his birth to "prepare the way of the Lord." He was "the voice of him that crieth in the wilderness, Prepare ye the way of the Lord, make straight in the desert a highway for our God." (Isaiah 40:3) When he was 8 days old he was ordained by an angel "to overthrow the kingdom of the Jews, and to make straight the way of the Lord before the face of his people, to prepare them for the coming of the Lord, in whose hand is given all power." (Doctrine and Covenants 84:28)

While John was not the first to administer the ordinance of Baptism, he was the only man having authority of God to do so. At that time, Israel had fallen away from the teachings of the prophets and although they were observing the Law of Moses, they were doing so without purpose or authority. Israel had not had a prophet with authority from God since the days of Malachi because of their rejection of God's word. Thus John the Baptist was called by God to prepare the Way for the Son of God, Jesus Christ, whom John knew to be the Son of God.

To learn more about Baptism and John the Baptist see

http://www.lds.org/scriptures/bd/baptism http://www.lds.org/scriptures/bd/john-the-baptist

Mikvah is associated with ritual purity.

Baptism is always associated with discipleship.

Israel was baptized into Moses's teachings.

John's baptism took disciples away from competing rabbis.

John considered Jesus's teachings to be superior to his own, requiring him to be baptised BY Jesus and to follow Him.

Paul was reluctant to baptize new believers because he thought it created division .

Why was the term baptism used to describe what the competing rabbis were doing?

Was it a case of using an equivalent Greek word to convey an existing Hebrew meaning? We know that Adonai, sarx, psyche, etc were used inspite of not being the exact equivalents. Or was it a word used to describe a new ritual or process.

Israel knew that they were still in exile, under judgment even though they were returned from Babylon. Jerusalem was under foreign rule, freedom to worship was controlled, the Temple had been desecrated several times. They searched the Scripture to discover what was needed to please God and be justified.

Each rabbi had a suite of laws they considered halakhic, required, for justification. Works of the law.

4QMMT and Paul: Justification, "Works" and Eschatology

• Quote

• The ‘works’ commended in MMT, then, are designed to mark out God’s true people present time , the time when the final fulfillment of Deuteronomy has begun but concluded. They are designed (C30) ‘so that you may rejoice words of ours to be true’. These extrabiblical commands will thus enable the sect to the verdict of the last day, when it will be clearly r a [118] the identity and context of the differentia of the in the is not yet at the end of time , finding these anticipate evealed that those who follow this particular halakhah are indeed the true, renewed people of God.

• (3) This brings us to the key comparison between MMT and Paul. Paul, arguably, held a version of the same covenantal and eschatological scheme of thought as MMT; but, in his scheme, the place taken by ‘works of Torah’ in MMT was taken by ‘faith’. Paul, like MMT, believed (a) in a coming ‘last day’ when all would be revealed, and (b) that the verdict of that last day could be anticipated in the present when so meone displayed the appropriate marks of covenant membership. For him, though, the appropriate marks were not ‘works’, either of the biblical Torah or of postbiblical halakhah, but faith: more specifically, faith in the God who raised Jesus from the dead.

[Source]

John taught the need to follow ALL the Law. Those who followed his teachings were inducted into his group by baptism. In 1st Century Palestine, the term baptism could have come from the word bapto or related words like baptizo. We have a 2nd Century BC document here containing both those words:

• Quote

• The clearest example that shows the meaning of baptizo is a text from the Greek poet and physician Nicander, who lived about 200 B.C. It is a recipe for making pickles and is helpful because it uses both words. Nicander says that in order to make a pickle, the vegetable should first be 'dipped' (bapto) into boiling water and then 'baptised' (baptizo) in the vinegar solution. Both verbs concern the immersing of vegetables in a solution. But the first is temporary. The second, the act of baptising the vegetable, produces a permanent change. When used in the New Testament, this word more often refers to our union and identification with Christ than to our water baptism. e.g. Mark 16:16. 'He that believes and is baptised shall be saved'. Christ is saying that mere intellectual assent is not enough. There must be a union with him, a real change, like the vegetable to the pickle! Bible Study Magazine, James Montgomery Boice, May 1989. Translated Words KJV (80)

[Source]

1 Peter 3:21-22 NET And this prefigured baptism, which now saves you – not the washing off of physical dirt but the pledge of a good conscience to God – through the resurrection of Jesus Christ, who went into heaven and is at the right hand of God with angels and authorities and powers subject to him.

In the NT, is baptism a ritual for induction to a course of indoctrination or the indoctrination itself. The answer is: both.

Induction brings one into the church. Indoctrination saves.

Israel was baptized into Moses (God's People/ecclesia) drank from the Rock (Christ) did not believe, resisted indoctrination, rebelled, perished.

The questions are:

Since the OT doesn't seem to speak of it and the NT jumps right in with everybody seeming to understand that such a practice exists (even if there is some question as to who/what/how/when), what happened in between the Testaments? Was baptism something that was done by anybody for any reason? If so by who and what for? Did the religious Jews of the time have such a practice?

I have proposed that baptism had two functions: to recognise participants of a covenant, and to a lesser extent, release the participants from their previous obligations.

In between the testaments there were no new covenants made, for which individuals needed to be recognised through baptism as participants. However, the Sinaitic Covenant had been contaminated. God required Israel to perform all the requirements of the law, the minor ones, circumcision, Sabbath keeping and dietary rules, as well as the weightier requirements, justice, mercy and faithfulness. This had devolved into Jews keeping only those rules that identified them as children of Abraham. John corrected them: Abraham was recognised as a friend of God not by circumcision (at the time not required) but by his loyalty, believing God's words. Now Israel must show that same loyalty, by believing God required all His law to be followed. So John's baptism was a baptism between the testaments, calling for Israel to repent, have a change of heart, to revert to the original meaning of the Sinaitic Covenant.

Did the religious Jews of the time have that practice?

If you analyse the situation, all the rabbis were misinterpreting the Old Covenant. They all searched the Scripture looking for the perfect markers of being the children of Abraham. John was the only corrective to this misunderstanding. For the Abrahamic Covenant, justification, identification as participants, was belief that the Sinaitic Covenant required all the law to be followed. Baptism, recognition as particants in the Abrahamic Covenant was given on being faithful to the Sinaitic Covenant, showing fruit indicating agreement to it, changing their existing mindset, repenting, meta-noia-ing.

When God instituted the Levitical priesthood, Moses washed (baptized fully) Aaron and his sons into said priesthood (Ex. 29:4, 40:12). So yes, baptism was God commanded, as a one-time event upon the start of a priesthood and subsequently by Levitical descendants into that priesthood.

John the Baptist's job, as the forerunner of Messiah, was to turn the hearts, make straight the paths, and turn the disobedient to the just (Mal. 3:1). He baptized a repentance (change the mind) for the remission of sins. This change of course was completely foreign to a people who were instructed to go to the Temple for sacrifices for their sins. Was the baptism of John from heaven or men? Heaven obviously.

What was happening was the same type of groundwork for John to baptize Jesus into the Melchizedek priesthood. Like Moses with Aaron, Jesus did not appoint Himself, but fulfilled all righteousness (Heb. 5:4).

So, baptism in the NT has a myriad of meanings. One that was lost is the pattern of one-time water baptism into the priesthood, not of Levi, not for salvation, but of Melchizedek, commanded by God, of believers into Christ, with Christ as our High Priest.

PS. To add some commentary from others about the one-time washing (baptism) of Aaron and sons as akin to the washing of Christians.

David Guzik

c. And you shall wash them with water: The process of consecration began with cleansing. All priestly ministries began with cleansing, and a cleansing that was received: you shall wash them. Aaron and his sons did not wash themselves; they received a washing.

i. This was humbling, because it took place publicly at the door of the tabernacle of meeting. We cannot be cleansed from our sin without being humbled first.

ii. This great cleansing was a one-time thing. From then on they just needed to cleanse their hands and their feet.

iii. Like these ancient priests, every Christian is washed by the work of God’s word (Ephesians 5:26), by the regenerating work of the Holy Spirit (Titus 3:5). This cleansing work was accomplished by the death of Jesus for us (Revelation 1:5) and appropriated by faith. Matthew Henry

The ceremonies wherewith it was to be done were very fully and particularly appointed, because nothing of this kind had been done before, and because it was to be a statute for ever that the high priest should be thus inaugurated. Now, (1.) The work to be done was the consecrating of the persons whom God had chosen to be priests, by which they devoted and gave up themselves to the service of God and God declared his acceptance of them; and the people were made to know that they glorified not themselves to be made priests, but were called of God, Heb. 5:4, 5. ... [1.] They were to be washed (v. 4), signifying that those must be clean who bear the vessels of the Lord, Isa. 52:11. Those that would perfect holiness must cleanse themselves from all filthiness of flesh and spirit, 2 Co. 7:1; Isa. 1:16-18. They were now washed all over; but afterwards, when they went in to minister, they washed only their hands and feet (ch. 30:19); for he that is washed needs no more, Jn. 13:10. Robert Jamieson

wash them with water. And. . . take the garments--The manner in which these parts of the ceremonial were performed is minutely described, and in discovering their symbolical import, which indeed, is sufficiently plain and obvious, we have inspired authority to guide us. It signified the necessity and importance of moral purity or holiness ( Isa 52:11 Jhn 13:10 2Cr 7:1 1Pe 3:21 ).

To answer the question, No, during the Old Testament period the Mosaic Law didn't require immersion rituals, but sprinkling (of blood) & washing (bronze laver) are covered. Elisha/Naaman account records 'dipping in Jordan (2kings5:14)'; however it was not a practiced ritual but could have evolved from the rituals presented then. But the mission of John the Baptist was to 'Baptize' and hence Baptism before Christ was introduced only by John; how & when he got this command from God is not recorded in the bible.

I believe John's act was to pave the way for transition from the law that required temple sacrifice 'without repentance' to the law of spirit that requires 'personal repentance'. That's why the bible says in Mal3:1 'I will send my messenger who will prepare the way before me - underline 'prepare the way' - the hearts & minds of temple ritualistic Jews. John himself has testified that Jesus will baptize with the Holy Spirit - after of course the repentance & washing away of sins by the blood of Christ Jesus.

You may note after Jesus's baptism & temptation in the wilderness, when Jesus began His preaching John had been thrown in prison (Mat4:12). I think, by then John's Job was over and God didn't allow overlapping of ministries by John & Jesus. Mat4:17 says Jesus began preaching repentance, thus started the message of transformation to the law of spirit.

John's Baptism for the remission of sins is particularly interesting, because he seems to have performed it in place of the temple rites of purification via sin offerings. IE, he was competing with the priests in the temple (he was a priest himself, as was his father Zechariah, who came from an Aaronic family and was high enough to be able to offer incense sacrifices).

I think this means that John made the forgiveness of sins available to those who could not afford the costly temple sacrifices (animals had to be inspected and found ritually pure before being offered--and such animals were sold in the Temple precincts and were expensive); and John's location along the Jordan was more easily accessed by many of the poorer country folk.

John's baptizing in the Jordan must also have recalled the entrance into the Promised Land made by Joshua--after they crossed the river Jordan, Joshua set up 12 stone pillars--as the generation of their father at Sinai--a sign of the renewing of the covenant. Being baptized in the Jordan by John after confessing one's sins was thus a statement of affirmation that one was renewing, on a personal level, the covenant with God.

But it is the practice of requiring the confession of sins that is particularly interesting. As one offered a sin offering in the temple, it was necessary to mention to the priest that one was making the offering because of having committed a particular sin. John takes over this oral confession of sins in his baptizing.

Here, I believe, is the origin of the practice of auricular confession within the Apostolic Churches (those directly descended from the Apostles, namely the Orthodox and Catholic Churches). Jesus sent His apostles (many of whom had followed John) out to baptize--and from Acts there is no mention of detailing one's sins; this practice was reserved for sins committed after Baptism--it is the origin of the Sacrament of Penance or Reconciliation; and the whole question of a second forgiveness of sins (ie of those committed after Baptism) became a huge subject of debate in the Roman Church in the early Third century. The moral lenient position, allowing for forgiveness of serious sins committed after Baptism, won out.

The ‘rite’ which the Baptism of John  used was not new at all, or limited to sects, but was, based on Old Testament teaching and mainstream rabbinic tradition, however, John used it in an entirely different way. The rite, in the way John used it, fully mirrored his preaching, one of repentance.   In the Old Testament those who had contracted Levitical defilement were to ‘immerse’ before offering sacrifice. The symbol goes as far back as Moses, leading Israel through the waters of the Red Sea before the Law was even given. In the Law, for example if anyone touches an unclean bed because a man had a discharge on that bed, they must ‘wash their clothes and bathe with water, and they will be unclean till evening.’ (Leviticus 15:5). There were many such ritualistic cleanings through bathing and these became formal in various ways of immersion under the guidance and administration of priests. It was not called Baptism in the Old Testament though outwardly they are the same thing.
  According to the famous evangelical historian and Jew, ‘Alfred Edershem’, who taught Jewish history in the University of Oxford, that Gentiles who became ‘proselytes of righteousness,’ or ‘proselytes of the covenant’ were to be admitted to ‘full participation in the privileges of Israel by the threefold rites of circumcision, baptism, and sacrifice - the immersion being, as it were, the acknowledgment and symbolic removal of moral defilement, corresponding to that of Levitical uncleanness.’.   Therefore, although it had never before been proposed that Israel should undergo a ‘baptism of repentance,’ like the Gentiles, John was using an existing rite in a dramatic way to ensure the way was clear for Messiah, by repentance and readiness for the gospel of forgiveness. It was only in preparation of Messiah, as a forerunner should be. Even the righteous under this baptism must consider themselves as Gentiles at this most historic transition from law to gospel.
  There was some debate whether this full immersion Baptism, used to convert Gentiles, ‘predated Christ’ but Alfred Edersheim provides ample proof in his appendix on the subject, in the book 'The Life and Times of Jesus the Messiah'- Appendix XII. He quotes that the subject was actually debated by Hillel and Shammai (the great competing schools of traditionalism at the time of Christ). Shammai allowed proselytes to partake Passover ‘after baptism’ but Hillel forbid it.  

Etymology

The English word “baptism” is a loanword derived from the Greek words βάπτισμα and βαπτισμός.¹ Both of these Greek nouns are related to the Greek verb βαπτίζω, from which is derived the English verb “baptize,” also a loanword.

In A Comprehensive Etymological Dictionary of the English Language, Ernest Klein wrote,²

The Hebrew Verb טָבַל and Noun טְבִילָה

The Hebrew equivalent of the English noun “baptism” and the Greek noun βαπτισμός is the noun טְבִילָה (tevila), which is related to the verb טָבַל (taval), essentially meaning “to dip.”

Heinrich Friedrich Wilhelm Gesenius wrote,³

While the noun טְבִילָה does not occur in the Hebrew Tanakh, it does occur often in the Mishna and Gemara (a.k.a. Talmud) and contemporary Jewish literature.

Regarding the noun טְבִילָה, Marcus Jastrow wrote,⁴

The Hebrew verb טָבַל occurs 16 times in 16 verses in the Hebrew Tanakh. It is translated in the 1769 ed. of the King James Version as follows: dip (15x), plunge (1x).

The following table lists the translation in the LXX of each occurrence of the Hebrew verb טָבַל in the Masoretic text:

Verse Hebrew TextGreek Text
Gen. 37:31 וַיִּטְבְּלוּ ἐμόλυναν
Exo. 12:22 וּטְבַלְתֶּם βάψαντες
Lev. 4:6 וְטָבַל βάψει
Lev. 4:17 וְטָבַל βάψει
Lev. 9:9 וַיִּטְבֹּל ἔβαψεν
Lev. 14:6 וְטָבַל βάψει
Lev. 14:16 וְטָבַל βάψει
Lev. 14:51 וְטָבַל βάψει
Num. 19:18 וְטָבַל βάψει
Deu. 33:24 וְטֹבֵל βάψει
Jos. 3:15 נִטְבְּלוּ ἐβάφησαν
Ruth 2:14 וְטָבַלְתְּ βάψεις
1 Sam. 14:27 וַיִּטְבֹּל ἔβαψεν
2 Kings 5:14 וַיִּטְבֹּל ἐβαπτίσατο
2 Kings 8:15 וַיִּטְבֹּל ἔβαψεν
Job 9:31 תִּטְבְּלֵנִי ἔβαψας

The Hebrew verb טָבַל and its conjugations are consistently translated by a conjugation of the Greek verb βάπτω in 14 of its 16 occurrences, or approximately in 88% of its occurrences. It is translated by a conjugation of the Greek verb βαπτίζω once, in 2 Kings 5:14, and by a conjugation of the Greek verb μολύνω also once, in Gen. 37:31.

The Greek Verbs βάπτω and βαπτίζω

In the LXX

The Greek verb βάπτω occurs 16 times in 16 verses in the LXX. However, unlike טָבַל, it does not occur in Gen. 37:31 or 2 Kings 5:14, but it does occur in Lev. 11:32 wherein it translates a conjugation of the Hebrew verb בָּא (ba), as well as Psa. 67:24⁵ wherein it translates a conjugation of the Hebrew verb מָחַץ (machatz).

On the other hand, the Greek verb βαπτίζω occurs 4 times in 4 verses in the LXX: 2 Kings 5:14, Isa. 21:4, Judith 12:7, and Sirach 34:25. In Isa. 21:4, it is used in the sense of “overwhelm” to translate a conjugation of the Hebrew verb בָּעַת (baʿat).⁶

In the Greek New Testament

The Greek verb βάπτω occurs 3 times in 3 verses in the 1550 Textus Receptus, and it is translated in the 1769 ed. of the KJV in all 3 occurrences by the English verb “dip.”⁷ On the other hand, the Greek verb βαπτίζω occurs 86 times in 65 verses in the 1550 Textus Receptus. It is predominately translated by a conjugation of the English verb “baptize.”

On the distinction between the verbs βάπτω and βαπτίζω, perhaps most often quoted is the physician Nicander of Colophon (Νίκανδρος ὁ Κολοφώνιος; 2nd c. B.C.), who himself was cited by Athenaeus in The Deipnosophists (Δειπνοσοφισταί). Athenaeus wrote,⁸

But they also ate as an appetizer turnips done in vinegar and mustard, as Nicander plainly shows in the second book of the Georgics; for he says: “Of turnip and cabbage, in truth, two families appear in our gardens, long and solid. The latter you wash and dry in the north wind, and they are welcome in winter even to the idle stay-at‑homes; for soaked in warm water they come to life again. But the other, the turnip roots, you cut in thin slices, gently cleaning away the undried outer skin, and after drying them in the sun a little, either dip (ἀποβάπτων) a quantity of them in boiling water and soak (ἐμβάπτισον) them in strong brine; or again, put equal parts of white must and vinegar in a jar together, then plunge the slices in it, having dried them off with salt.”

ὅτι δ᾽ ἤσθιον διὰ ἀναστόμωσιν καὶ τὰς δι᾽ ὄξους καὶ νάπυος γογγυλίδας σαφῶς παρίστησι Νίκανδρος ἐν δευτέρῳ Γεωργικῶν λέγων οὕτως: γογγυλίδος δισσὴ γὰρ ἰδ᾽ ἐκ ῥαφάνοιο γενέθλη μακρή τε στιφρή τε φαείνεται ἐν πρασιῇσι. καὶ τὰς μὲν θ᾽ αὕηνον ἀποπλύνας βορέῃσι, προσφιλέας χειμῶνι καὶ οἰκουροῖσιν ἀεργοῖς: θερμοῖς δ᾽ ἰκμανθεῖσαι ἀναζώουσ᾽ ὑδάτεσσι. τμῆγε δὲ γογγυλίδος ῥίζας （καὶ ἀκαρφέα φλοιὸν ἦκα καθηράμενος） λεπτουργέας, ἠελίῳ δὲ αὐήνας ἐπὶ τυτθὸν ὁτὲ ᾿ν ζεστῷ ἀποβάπτων ὕδατι δριμείῃ πολέας ἐμβάπτισον ἅλμῃ, ἄλλοτε δ᾽ αὖ λευκὸν γλεῦκος συστάμνισον ὄξει ἶσον ἴσῳ, τὰς δ᾽ ἐντὸς ἐπιστύψας ἁλὶ κρύψαις.

The History of Pre-Christian Baptism or Immersion

Immersion for Restoring Cleanness/Purity (טָהֳרָה)

Sirach 34:25 references the practice of immersion or “baptism” when it describes those who touched corpses as later baptizing themselves or being baptized (βαπτιζόμενος).

If a man washes after touching a dead body, and touches it again, what has he gained by his washing? ^RSV

βαπτιζόμενος ἀπὸ νεκροῦ καὶ πάλιν ἁπτόμενος αὐτοῦ, τί ὠφέλησεν ἐν τῷ λουτρῷ αὐτοῦ

According to the Torah, an Israelite who contacts a corpse is rendered unclean for seven days. In Num. 19:11, it is written,

He who touches the dead body of any soul of a man, then he shall be unclean seven days.

הַנֹּגֵעַ בְּמֵת לְכָל נֶפֶשׁ אָדָם וְטָמֵא שִׁבְעַת יָמִים

Moshe ben Maimon reiterated the process required to become clean (restore purity) after contacting a corpse (cp. Num. 19:12–19). In the Mishneh Torah, he wrote,⁹

How is a person, unclean from a corpse, made clean by the water of separation? A clean man takes three stalks of hyssop and binds them with one bond. Each stalk should have at least one bud. He should immerse the top of the buds in the water of separation while it is in a vessel, focus his intent, and sprinkle it on the [unclean] man or [unclean] vessels, on the third day and on the seventh day after sunrise. If one sprinkled after dawn, it is acceptable. After the water has been sprinkled upon it on the seventh day, he should immerse in a mikve during the day, wait until nightfall, and then, in the evening, he is clean.

כֵּיצַד מְטַהֲרִין טְמֵא מֵת בְּמֵי נִדָּה: לוֹקֵחַ אָדָם טָהוֹר שְׁלוֹשָׁה קְלָחִין שֶׁלְּאֵזוֹב, וְאוֹגְדָן אֲגֻדָּה אַחַת, וּבְכָל בַּד וּבַד גִּבְעוֹל אֶחָד. וְטוֹבֵל רָאשֵׁי גִּבְעוֹלין בְּמֵי נִדָּה שֶׁבַּכְּלִי, וּמִתְכַּוֵּן וּמַזֶּה עַל הָאָדָם אוֹ עַל הַכֵּלִים, בְּיוֹם הַשְּׁלִישִׁי וּבְיוֹם הַשְּׁבִיעִי, אַחַר שֶׁתָּנֵץ הַחַמָּה; וְאִם הִזָּה מִשֶּׁעָלָה עַמּוּד הַשַּׁחַר, כָּשֵׁר. וְאַחַר שֶׁיַּזֶּה עָלָיו בְּיוֹם הַשְּׁבִיעִי, טוֹבֵל בַּיּוֹם; וּמַעְרִיב שִׁמְשׁוֹ, וַהֲרֵי הוּא טָהוֹר לָעֶרֶב.

After being sprinkled with the “water of separation” (created with the ashes of the red heifer) on the third and seventh day of his uncleanness, the Israelite was to “wash his clothes and bathe in water” (וְכִבֶּס בְּגָדָיו וְרָחַץ בַּמַּיִם), and then he would be clean after sunset (cp. Num. 19:19). The Hebrew text does not use a conjugation of the verb טָבַל (taval) to describe the act of bathing in water, but rather, a conjugation of the verb רָחַץ (rachatz).

The Hebrew Verb רָחַץ

The Hebrew verb רָחַץ occurs far more often than טָבַל in the Hebrew Tanakh, 72 times in 71 verses. While it is often translated as “wash” and sometimes as “bathe,” the act of bathing consisted of immersing in a mikve (pool of water) in order to become clean after contracting uncleanness (i.e., becoming unclean).

The Mikve (מִקְוֶה)

In the Mishneh Torah, Moshe ben Maimon wrote,¹⁰

All unclean things - whether humans or vessels, whether they were defiled with a severe uncleanness according to the Torah, or whether they were defiled with uncleannesses according to the words [of the rabbis], do not become clean except by immersion (טְבִילָה) in water that is collected in a pool.

כָּל הַטְּמֵאִים--בֵּין אָדָם, בֵּין כֵּלִים, בֵּין שֶׁנִּטְמְאוּ בְּטֻמְאָה חֲמוּרָה שֶׁלַּתּוֹרָה, בֵּין שֶׁנִּטְמְאוּ בִּטְמָאוֹת שֶׁלְּדִבְרֵיהֶם--אֵין לָהֶן טַהְרָה, אֵלָא בִּטְבִילָה בְּמַיִם הַנִּקְוִים בַּקַּרְקָע.

The water that is collected in a pool is also known as a מִקְוֶה (mikve). There was, as can be expected, laws pertaining to the mikve and how to properly immerse therein.

In the Mishneh Torah, Moshe ben Maimon also wrote,¹¹

Everywhere that the bathing of one’s flesh or the washing of one’s garments from uncleannesses is stated in the Torah, it is nothing but the immersion of the entire body in a mikve.

כָּל מָקוֹם שֶׁנֶּאֱמָר בַּתּוֹרָה רְחִיצַת בָּשָׂר וְכִבּוּס בְּגָדִים מִן הַטְּמָאוֹת, אֵינוּ אֵלָא טְבִילַת כָּל הַגּוּף בְּמִקְוָה

Accordingly, immersion (“baptism") or טְבִילָה was required for lepers,¹² people and objects that contacted seminal fluid (e.g., during sexual intercourse or from a nocturnal emission),¹³ the nidda or menstruant,¹⁴ and so and so on. It is regarding all these baptisms or immersions that the author of the Epistle to the Hebrews wrote,¹⁵

concerned only with foods and drinks and various baptisms, and carnal ordinances imposed until the time of reformation.

μόνον ἐπὶ βρώμασιν καὶ πόμασιν καὶ διαφόροις βαπτισμοῖς καὶ δικαιώμασιν σαρκὸς μέχρι καιροῦ διορθώσεως ἐπικείμενα

Regarding the Greek phrase «διαφόροις βαπτισμοῖς» (“various baptisms”) in Heb. 9:10, Franz Delitzsch wrote,¹⁶

Immersion of a Convert/Proselyte to Judaism

Aside from being used to remove physical uncleanness (טֻמְאָה), baptism or immersion was one of three requirements for the conversion of a proselyte to Judaism.¹⁷

In the Babylonian Talmud, it is written,¹⁸

Derive from it that one is not a convert until he is circumcised and immerses.

וש"מ אינו גר עד שימול ויטבול

The requirement for immersion of a proselyte was derived from Exo. 24:8 since אין הזאה בלא טבילה, that is, “There is no sprinkling [of blood] without immersion [beforehand].”¹⁹ It is reasoned that when the mixed multitude entered the covenant, they had immersed before they were sprinkled with the blood by Moses. This conversion made one “like a newborn child.”²⁰

Footnotes

¹ There is also another noun, βάπτισις, that shares a similar meaning, although it occurs seldomly. See Josephus. Jewish Antiquities. Book 18, §117. Like English, Latin also possessed loanwords such as baptisma, baptismum, and baptismus.
² Vol. 1, p. 147
³ p. 317
⁴ p. 516–517
⁵ Psa. 68:23 in the KJV.
⁶ See BDAG, p. 165, βαπτίζω, 3. c.; LSJ, p. 305, βαπτίζω, under "transf."; Thayer, p. 94, βαπτίζω, I. 3.
⁷ There is also a related verb, ἐμβάπτω, which also occurs 3 times in 3 verses.
⁸ p. 183, Book IV, Ch. 11
⁹ Sefer Tahara, Hilkhot Para Aduma, Chapter 11, Halakha 1
¹⁰ Sefer Tahara, Hilkhot Mikvaot, Chapter 1, Halakha 1
¹¹ Sefer Tahara, Hilkhot Mikvaot, Chapter 1, Halakha 2
¹² Lev. 15:2–13
¹³ Lev. 15:16
¹⁴ Lev. 15:19–24
¹⁵ Heb. 9:10
¹⁶ p. 73
¹⁷ The three requirements are: circumcision, immersion, and offering a sacrifice (this requirement was only mandatory when the Temple existed).
¹⁸ Seder Nashim, Tractate Yevamot, Gemara, Chapter 4, Folio 46b
¹⁹ Seder Nashim, Tractate Yevamot, Gemara, Chapter 4, Folio 46b
²⁰ Seder Nashim, Tractate Yevamot, Gemara, Chapter 4, Folio 48b: רבי יוסי אומר גר שנתגייר כקטן שנולד, that is, "Rabbi Yose said, 'A proselyte that converts is like a newborn child."

References

Arndt, William; Bauer, Walter; Danker, Frederick William. A Greek-English Lexicon of the New Testament and Other Early Christian Literature. 3rd ed. Chicago: U of Chicago P, 2000.

Athenaeus. The Deipnosophists. Books VIII-X. Trans. Gulick, Charles Burton. Cambridge: Harvard UP, 1969.

Dale, James Wilkinson. Classic Baptism: An Iniquiry into the Meaning of the Word ΒΑΠΤΙΖΩ, as Determined by the Usage of Classical Greek Writers. Philadelphia: Rutter, 1867.

Dale, James Wilkinson. An Iniquiry into the Usage of ΒΑΠΤΙΖΩ, and the Nature of Judaic Baptism, as Shown by Jewish and Patristic Writings. 3rd ed. Philadelphia: Rutter, 1873.

Delitzsch, Franz. Commentary on the Epistle to the Hebrews. Vol. 2. Trans. Kingsbury, Thomas L. Edinburgh: Clark, 1872.

Gesenius, Heinrich Friedrich Wilhelm. Gesenius’s Hebrew and Chaldee Lexicon to the Old Testament Scriptures. Trans. Tregelles, Samuel Prideaux. London: Bagster, 1860.

Jastrow, Marcus. A Dictionary of the Targumim, the Talmud Babli and Yerushalmi, and the Midrashic Literature. London: Luzac; New York: Putnam, 1903.

Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. Vol. 1. New York: Elsevier, 1966.

Liddell, Henry George; Scott, Robert; et al. A Greek-English Lexicon. 9th ed. Oxford: Clarendon, 1940.

Moshe ben Maimon. Mishneh Torah (מִשְׁנֵה תּוֹרָה). Ed. Mechon-Mamre. Jerusalem: Mechon-Mamre, 2015.

Thayer, Joseph Henry. A Greek-English Lexicon of the New Testament: Being Grimm Wilke’s Clavis Novi Testamenti. Rev. ed. New York: American Book, 1889.

Ritual cleansing was a common part of some Jewish sects around the turn of the era. One of the best examples of this (that I know of) comes from Khirbet Qumran (where the Dead Sea Scrolls were found)—where there were found several miqvot (sn. miqveh) used for a sort of "baptism." It is commonly held that the people living at Qumran were Essenes, and some have speculated that John the Baptist may have been a part of this group or one similar to it.

The Qumran community began slightly before the turn of the age and continued into the early 1st century CE, so it was roughly contemporaneous with Jesus. For a really nice intro to the Archaeology of Qumran, see Jodi Magness' The Archaeology of Qumran and the Dead Sea Scrolls.

There is a ton of literature from and about the "inter-testamental" period (usually referred to as the "Second Temple Period" by scholars). I'll point you to the great intro to the corpus by George Nickelsburg, Jewish Literature Between The Bible And The Mishnah.