Why did Jesus use the expression, "very truly I tell you"?

The answer is simple, it is to remind us that He is the Truth. It is a living testament that He was and He is the Truth and He was telling the truth yet people of Israel, not necessarily Jews, were blind and unable to see or hear or accept the Truth, Jesus Christ. It has nothing to do with English and or phrases in any language. "I am telling you the truth!"

To put things a bit into perspective, for example if your mom tells you 1000000000 times: "Do not touch the fire because you are going to burn yourself. I am telling you the truth!" and you still do it and you burn yourself was she telling you the truth? Well yes, she told you so!

Jesus Christ the Son of God was emphasizing the goodness of truth in itself and if you look at scripture it says in John 14:6 that Jesus "literally", incarnate, in human form, is absolute truth. It also says that the only way to God and Heaven is to accept Him as savior.

Throughout the old and new testaments the goodness of truth is emphasized and the fact that a lie was the beginning of all evil is well, the opposite of good ... bad!

So, I agree with Aleks answer also on this.

Verily, verily I Say Unto You This phrase is used with two "verily's" only in John's Gospel. And with just one "verily" in the Synoptics.

It is important to note that congregations and rabbis are in the custom to saying amen at the "end" of a prayer, lecture, or pertinent statement. This acknowledges agreement with the statement of someone else.

However Jesus used this phrase at the beginning of His statements to underscore the fact that He was speaking with authority. He was not relying on anyone else. Such that all the people noted a difference between His teaching and that of the learned rabbis!

When Jesus had finished saying these things, the crowd were amazed at His teaching because He taught as one who had authority, and not as their teachers of the Law. (Matthew 7:29, Mark 1:22)

This phrase was not just a colloquial expression, common by all. It was a phrase that set Jesus apart as, not just another teacher of morality, but the teacher sent by God (John 3:2, Nicodemus got it wrong!)

A word of caution was given by the writer of Hebrews, that must be taken seriously by all:We must pay more careful attention to what we have heard, so that we do not drift away. For if the message spoken by angels was binding, and every violation and disobedience received its just punishment, how shall we escape if we ignore such a great salvation? This salvation, which was first announced by the Lord, was confirmed to us by those who heard Him. God also testified to it by signs, wonders and various miracles, and gifts of the Spirit distributed according to His will! (Hebrews 2:1-4)

In the Garden of Eden, the devil when tempting the children of God said "did God really say...?". The children of God doubted the truth of God and sinned. Jesus was dealing with that decision to not believe God's truth. Is it not underscored when Jesus says, "I am the way, the TRUTH, and the life"? TRUTH is part of his name and he simply was re-iterating, in essence, I told you the TRUTH in the Garden, I tell you the TRUTH today, and I will tell you the TRUTH tomorrow. Seems as if we learn best by repetition. So he repeated his truth constantly. Just before he ascended, he said 'I go to prepare a place for you, if it was not so I would have told you so'. Once more, affirming that he tells only the TRUTH.

Jesus used the expression, "I tell you the truth" or "Verily, Verily, I say unto you" to say to those to whom he was speaking, "Listen to me (give me your full attention), what I am about to say to you is of special importance." Jesus did not need to lie, he was and is the truth. He was a Jewish Rabbi and Teacher of the New Covenant (New Testament). He was and is the Messiah. The living Christ and Son of God almighty.

This word shows authority. Jesus is an "ambassador" talking to us about God's kingdom. Jesus was fulfilling God's purpose here on earth. He was on a mission. Jesus has said many times that he does not speak on his own authority but for the one who sent him. John 12:49. John 8:42. John 17:17
Sanctify them by the truth; your word is truth.

We understand that God's word is the truth and Jesus is speaking with authority from God. Therefore whenever Jesus says "I tell you the truth" he is Quoting God's word, not his word.

He was just using it as an expression to make it known that whatever he was about to say needs to be taken as seriously as possible. Jesus has always told the truth but I cannot say if everything he said was taken seriously by the people he was speaking to. So it could have been used more in the context of "listen carefully to what I am about to tell you now" or "Pay attention!".

The lines spoken by Jesus to Pilate come to mind for some reason when I read our question.

"That is why I was born. To give testimony to the truth. All men who hear the truth hear my voice."

The answer to this question is simple. Read John 18:37... remember, the purpose of Jesus’ coming and His birth was to “bear witness to the truth.” Every time Jesus spoke or taught, it was about releasing truth. His word shed light of knowledge which opened the eyes of the spiritually blind. If you read John, he says “truly, truly I tell you..” Why does He put premium to truth? Because the world is a slave, in a bondage... of sin, Satan, lies of tradition, culture etc etc. word of truth brings light exposing thereby the lies underneath, releasing the people.

When Jesus says "I tell you the truth" the Greek word "amen" is being used. Amen can be translated as "I tell you the truth," "verily," and "so be it" or "let it be.". We have difficulty translating it into English, and the reason it is so odd is because we don't use those phrases colloqioully today.

I hope this helps!

The Phrase

As retained in the Latin Vulgate, and therefore the Douay-Rheims ('the Catholic KJV'), but usually obscured in most English translations, Jesus is actually saying the vast, vast majority of the time, "Amen, Amen I say to you."

Which does mean 'Truly, I tell you a (solemn or sure or serious) truth (i.e. so pay special attention)'.

In Greek, the authors have transliterated "Amen" into Greek (authors who also recorded Him say "I tell you the truth," implying they saw it important to at least record that He did say "Amen" specifically—John records that Jesus in fact said 'Amen' twice in each instance, "Amen, Amen": since he didn't put extra words into Jesus' mouth, the other authors have determined, not erroneously, this to be perhaps lost on their audience, and so retained the term, but not the exact words).

For example:

Luke 4:20-30

And he said: Amen I say to you, that no prophet is accepted in his own country. In truth I say to you, there were many widows in the days of Elias [Elijah] in Israel, when heaven was shut up three years and six months, when there was a great famine throughout all the earth. And to none of them was Elias sent, but to Sarepta of Sidon, to a widow woman. And there were many lepers in Israel in the time of Eliseus the prophet: and none of them was cleansed but Naaman the Syrian. And all they in the synagogue, hearing these things, were filled with anger. And they rose up and thrust him out of the city; and they brought him to the brow of the hill, whereon their city was built, that they might cast him down headlong. But he passing through the midst of them, went his way.

The Word

"Amen" is a Hebrew/Aramaic (getting it from Hebrew) word meaing "May it be" or "So be it." Since this is a rather loose translation of the word, which usually followed the end of prayer¹ (a faithful assent to the prayer), it means here, in Jesus' use, something close to "Yes, truly, ..." or "Most assuredly, it shall be no other way than..."

The Catholic Encylopedia (Catholic Encylopedia 1914, A, "Amen") says the following:

"So frequent was this Hebrew in the mouth of Our Saviour", observes the Catechism of the Council of Trent, "that it pleased the Holy Ghost to have it perpetuated in the Church of God". In point of fact St. Matthew attributes it to Our Lord twenty-eight times, and St. John in its doubled form twenty-six times. As regards the etymology, Amen is a derivative from the Hebrew verb aman "to strengthen" or "Confirm".

So Jesus is highlighting those truths which He deems most important to be remembered or believed, not implying these are true and the rest of what He says is not true—God forbid. Jesus also appears to be the only Person to use "Amen" before what He says or affirms as most true, which is interesting.

1 ^{Neh 8:6; cf. Judith 13:26—this seems to be a double expression of the usual one "Amen!" (1 Chron 16:36), and not the sense in which Jesus is using "Amen." That is to say, 'A solemn or sure truth is about to be spoken:'}

"The Amen"

Jesus is said to be "the Amen." (ὁ Aμην ho amEn) Jesus is the solemn Truth (John 14:6).

Revelation 3:14

And to the angel of the church of Laodicea, write: These things saith the Amen, the faithful and true witness, who is the beginning of the creation of God:

Amen in the context of Jesus is here tied to true/reliable witness (as far as I see it).

My Humble Opinion

When Jesus uses two "Amen"s, I believe He is actually showing that the Father affirms and approves and bears witness (represented by the first "Amen") that all He (represented by the second "Amen") says is most true and reliable. Not out of any necessity (John 8:13-14) but because it's something that's important to know for us who are not Jesus (John 8:14) and need to have witness from the Father:

John 5:31-32

If I bear witness of myself, my witness is not true.² There is another that beareth witness of me; and I know that the witness which he witnesseth of me is true.

2 ^{That is, true witness—valid witness for all intents and purposes; as far as we are concerned}

The fact that St. John's Gospel is focused on and written to highlight the relationship between the Father and the Son, I find this not impossible, and quite probable.

John 16:7

However, in John 16:7 He actually says, "εγω την αληθειαν λεγω υμιν.." which means "I tell you the truth.." but could just as easily be translated, equally so, though it would not be normal for anyone to say this except Jesus, "I, the Truth, tell you.."

However, He is still emphasizing that what He says will certainly come to pass (John 16:4).

When Jesus says "I tell you the truth", he does it to indicate that what he is telling is the truth.

John 23:42-43 (NIV)
42 Then he said, “Jesus, remember me when you come into your kingdom.” 43 Jesus answered him, “Truly I tell you, today you will be with me in paradise.”

Often times, Jesus speaks and replies using parables

For example, in Luke 10:25-37, a Pharisee asked him “Who is my neighbor?” Jesus replied with the parable of the Good Samaritan.

However, when Jesus said "I tell you the truth", he was indicating that he was not speaking in parables, but telling the truth.