You turn into a Christian the same way anyone else turns into a Christian-by being born again. Jesus put it this away:
Jesus answered and said to him, “Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.” How to be born again? The Bible tells us this also: He came to His own, and those who were His own did not receive Him. But as many as received Him, to them He gave the right to become children of God, even to those who believe in His name, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God.
This is how one turns into a Christian.