Define "Christianity."
"Define 'Christianity.'"
St. Paul teaches us in Galatians that Christianity is the acceptance of the belief that Jesus was the Messiah, that he died for our sins, and, through the power of God, defeated death, thereby giving the promise of eternal life to all those who accept that gift.
In Romans and other letters, St. Paul instructs us not to bicker about the denominational details (which is basically anything more than the first paragraph).