c. 1300, cristente, "Christians as a whole; state of being a Christian; the religion founded by Jesus," from Old French crestienté "Christendom; spiritual authority; baptism" (Modern French chrétienté), from Church Latin christianitatem (nominative christianitas), noun of state from christianus (see Christian). Gradually respelled to conform with Latin. Christendom is the older word for it. Old English also had cristennes.
Old English cristendom "Christianity, state of being a Christian, profession of faith in Christ by baptism," from cristen (see Christian) + -dom, suffix of condition or quality. The native formation, crowded out by Latinate Christianity except in the sense of "lands where Christianity is the dominant religion" (late 14c.). Similar formations are found in Scandinavian languages.