Of all of the numerous passages that could be presented as a short definition of Christianity, perhaps the best one is found in Colossians:
Christ in you, the hope of glory. (Col 1:27)

Christianity is CHRIST IN US, or if you prefer, Paul often speaks of US being IN CHRIST. These are the same Truth. There is certainly much that could be added as to how Christ comes to dwell in the believer, and of the Christian walk that emerges from His presence in us. But in the end, our personal Christianity is founded upon whether Christ dwells in us. If Christ is in us, we are a Christian. We are saved. We are on a great spiritual journey. Everything about our Christianity – indeed, everything about the hope of eternal glory – depends upon Christ in us.
Jesus Christ did not come to give us a new, or better, form of religion. He did not come to replace law with grace. Neither did He come to begin a movement. Rather, Jesus came to give us HIMSELF. “Christ in you,” is, “the mystery which has been hid from ages and from generations, but now is made manifest to his saints.” (Col. 1:26) “Christ in you,” is Christianity, the new birth, and the essence of the New Covenant. God has made Himself ONE with human beings through His Son, Jesus Christ.

In him was life. (John 1:4)