What did the early church fathers (pre-5th c.) teach with regards to the doctrine of baptism as a necessity for salvation?

The early Church fathers have addressed baptism in the writings I added to the comments above. They do not however attemp to defend it as Catholics today need to in the aftermath of the Protestant Seperation. There was no need. The necessity of Baptism as with other Traditions, were understood organically in the early Christian Culture by repetition and practice.

The method of Baptism was written in th Dideche and the Tradition preserved by the living body of Christ.

Concerning baptism, baptize in this way. After you have spoken all these things, "baptize in the name of the Father, and of the Son, and of the Holy Spirit," in running water. Didache Ad 50-100

Didache translates to "Teachings". The first line of this document reads "The Teaching of the Lord to the Gentiles, by the twelve Apostles"

Baptism of the New Covenant is the fulfillment of Circumcision of the Old Covenant. In the Old Covenant a person entered into the Grace (being saved), freely given to the Jews, and maintained that covenant relationship (stayed saved) by fallowing the works of the Law and the worship ordinances given to Moses. If they followed the law and were faithful to God, they died in a state of Grace and were saved by the blood of Christ.

Now, having entered into the New Covenant of Grace, freely given, by Baptism (being saved) the disciple of Christ maintains his relationship (stays saved) by participating in the Church by the Sacrements instituted by Christ.(Staying saved) (Note: not by works of the Law)then Persevering to the end In a life of repentance worship obedience and faith in Christ, and working in Christ as a member of his body. The Christian remains saved. Continues to be sanctified and finally is glorified. As long as he preservers to the end.

Understanding this relationship entered into not by genealogy but by Baptism via Faith for the convert, the early fathers unanimously explain in this Three fold example how baptism saved you (initially) by faith if you are a convert by Grace alone if you are an infant. Then after you enter into that family relationship you remain in it (maintain) by the Sacrements and complete your salvation by persevering to the end (achieved).

I don't suppose you can get any earlier than Paul for an "early church father". Here is what he says in Acts 19:2-6 (KJV):

He said unto them, Have ye received the Holy Ghost since ye believed? And they said unto him, We have not so much as heard whether there be any Holy Ghost. And he said unto them, Unto what then were ye baptized? And they said, Unto John's baptism. Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus. When they heard this, they were baptized in the name of the Lord Jesus. And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.

There's no mention of water baptism here. He's basically saying the action doesn't matter as much as the belief in Jesus Christ.

“First of all it bids us bear in mind that we have received baptism for the remission of sins, in the name of God the Father, and in the name of Jesus Christ, the Son of God, who was incarnate and died and rose again, and in the Holy Spirit of God. And that this baptism is the seal of eternal life, and is the new birth unto God, that we should no longer be the sons of mortal men, but of the eternal and perpetual God.”

-Irenaeus, Demonstration of the Apostolic Preaching Ch. 3

“Those, then, who, after having been baptized into Father, Son, and Holy Spirit, and having been taught that there is one divine nature in three subsistences, are rebaptized, these, as the divine Apostle says, crucify the Christ afresh (chapter 3)… But those who were not baptized into the Holy Trinity, these must be baptized again.” (chapter 9)

-John of Damascus, An Exposition of the Orthodox Faith, Book IV

“It is possible that a man is wishing to pay court to a woman, and came hither on that account. The remark applies in like manner to women also in their turn. A slave also perhaps wishes to please his master, and a friend his friend. I accept this bait for the hook, and welcome thee, though thou camest with an evil purpose, yet as one to be saved by a good hope. Perhaps thou knewest not whither thou wert coming, nor in what kind of net thou art taken. Thou art come within the Church's nets: be taken alive, flee not: for Jesus is angling for thee, not in order to kill, but by killing to make alive: for thou must die and rise again.” (5)

-Cyril of Jerusalem’s Catechetical Lectures, Lect. 1

“Great is the Baptism that lies before you: a ransom to captives; a remission of offences; a death of sin; a new-birth of the soul; a garment of light; a holy indissoluble seal; a chariot to heaven; the delight of Paradise; a welcome into the kingdom; the gift of adoption!” (16)

-Cyril of Jerusalem’s Catechetical Lectures, Lect. 1

“We may not receive Baptism twice or thrice; else it might be said, Though I have failed once, I shall set it right a second time: whereas if thou fail once, the thing cannot be set right…” (7)

-Cyril of Jerusalem’s Catechetical Lectures, Lect. 1

“Jesus sanctified Baptism by being Himself baptized. If the Son of God was baptized, what godly man is he that despised Baptism? But He was baptized not that He might receive remission of sins, for He was sinless; but being sinless, He was baptized, that He might give to them that are baptized a divine and excellent grace. For since the children are partakers of flesh and blood, He also Himself likewise partook of the same, that having been made partakers of His presence in the flesh we might be made partakers also of His Divine grace: thus Jesus was baptized, that thereby we again by our participation might receive both salvation and honor (11)… For thou goest down into the water, bearing thy sins, but the invocation of grace, having sealed thy soul, suffereth thee not afterwards to be swallowed up by the terrible dragon. Having gone down dead in sins, thou comest up quickened in righteousness. For if thou hast been united with the likeness of the Saviour's death, thou shall also be deemed worthy of His Resurrection. For as Jesus took upon Him the sins of the world, and died, that by putting sin to death He might rise again in righteousness; so thou by going down into the water, and being in a manner buried in the waters, as He was in the rock art raised again walking in newness of life (12)… Moreover, when thou hast been deemed worthy of the grace, He then giveth thee strength to wrestle against the adverse powers. For as after His Baptism He was tempted forty days…” (13)

-Cyril of Jerusalem’s Catechetical Lectures, Lect. 3