Why Did Jesus Say "Truly Truly" or "Verily Verily"?

In John 3, Nicodemus said to Jesus, Rabbi, we know you're a teacher come from God, for no one can do these signs that you do unless God is with him.

Jesus replied, Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.

Now that might seem like an odd reply since Nicodemus didn't mention the kingdom, but Jesus not only responded to Nick's statement, he declared his authority

Truly, or verily, in Greek is the word Amen. Yeah, that word we tack on to the end of prayers.

It means, so be it. For a gathering to conclude a prayer or preaching with Amen means to be in agreement with everything that was said.

But for Jesus to put this word at the start of his answer, it takes on a different meaning. When Jesus said, truly

I say to you, he was saying, I am a first -hand witness that what I say to you is the truth, as if he was there in the beginning when all truth was established.

Because he was. Jesus is the author of truth. John 1 .3 says, all things were made through him and without him was not anything made that was made.

Colossians 1 .16 says, for by him all things were created, whether thrones or dominions or rulers or authorities.

So when Nicodemus said, we know you're a teacher come from God, Jesus responded as if to say,

I am God, and no one gets to God unless he is born of God. In every gospel

Jesus says, truly I say to you, all divine claims to being the way, the truth, and the life, when we understand the text.