Christianity didn’t exist in the time of Noah.
Christ did.
Jesus very specifically included the account of NOAH in Christianity. He gave that as an example of how it would be again, when the next WORLD-WIDE JUDGEMENT comes down again, at the return of Jesus, himself.
AND ... in fact Jesus said that the entirety of the Old Testament (the law, prophets and writings) is about HIM.
So Jesus, Himself, made it all about Christianity.
I'd say that Christianity began in Genesis 3:15 when the promise was given to our fallen parents that there would be a Redeemer (the Messiah, aka the Christ, the Anointed One). The Seed of the Woman who would crush the head of the serpent, and free mankind from the curse of sin and death,
Through the ages, Mankind continued to look for Him - and Wise men found Him (as He said, 'if he seek me, you will find me').
So 'official' Christianity was ushered in at the Incarnation and Resurrection - but many were trusting in His finished work long before.
Here are some references to this ...
Matthew 24:37-39
37 But as the days of Noah were, so also will the coming of the Son of Man be.
38 For as in the days before the flood, they were eating and drinking, marrying and giving in marriage, until the day that Noah entered the ark,
39 and did not know until the flood came and took them all away, so also will the coming of the Son of Man be.
—
Luke 24:
25 Then He said to them, “O foolish ones, and slow of heart to believe in all that the prophets have spoken!
26 Ought not the Christ to have suffered these things and to enter into His glory?”
27 And beginning at Moses and all the Prophets, He expounded to them in all the Scriptures the things concerning Himself.
...
44 Then He said to them, “These are the words which I spoke to you while I was still with you, that all things must be fulfilled which were written in the Law of Moses and the Prophets and the Psalms concerning Me.”
45 And He opened their understanding, that they might comprehend the Scriptures.
46 Then He said to them, “Thus it is written, and thus it was necessary for the Christ to suffer and to rise from the dead the third day,
47 and that repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem.
—
And also, the prophetic Word from God, in the Garden of Eden about the coming one who is to provide salvation to the world and to all of creation ...
Genesis 3:14, 15
14 So the Lord God said to the serpent: “Because you have done this, You are cursed more than all cattle, And more than every beast of the field; On your belly you shall go, And you shall eat dust All the days of your life.
15 And I will put enmity Between you and the woman, And between your seed and her Seed; He shall bruise your head, And you shall bruise His heel.”