Synonyms for Christianity (same or very similar meaning)
WordNet sense 1 (the collective body of Christians throughout the world and history (found predominantly in Europe and the Americas and Australia)):
ChristendomWordNet sense 2 (a monotheistic system of beliefs and practices based on the Old Testament and the teachings of Jesus as embodied in the New Testament and emphasizing the role of Jesus as savior):
Christian religionFrom the ODE community, based on WordNetadd/edit