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.
In reality, John Chapter 1 is its real begining.
In “prophecy” yes. The real start of the Christian faith is John 1.
Genesis 1:26 contains the first reference to Christ, indirectly, as God refers to Himself in the plural:
“And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
As for the first Christians, Acts 11:26 is the first reference to Christians actually being called by that name, at Antioch.