If you simply want to know more of how Christianity was expressed in history, a knowledge of that progression is listed in the link I provided.
If your interested in knowing what it means to be a “Christian”, look to the one who defined it and exemplified it……Jesus.
John 3:3. “Jesus answered and said to him, “Most assuredly, I say to you, unless one is born again, he cannot see the kingdom of God.”
U.T. 