Christianity
Middle English: from Old French crestiente, from crestien ‘Christian’, influenced by late Latin christianitas, from Latin Christianus, from Christus ‘Christ’.
wiktionary
From Middle English Cristiente, Cristente, borrowed from Old French crestienté, from Medieval Latin stem of Chrīstiānitās, from Latin christianus, Christianus, from Ancient Greek Χριστιανός(Khristianós), from Χριστός(Khristós, “Christ, anointed one”) + Latin -anus(“suffix for of, related to”) + one more suffix borrowed from Latin "ity" makes the final Christian + -ity. The term was respelled in the early modern English period to more closely reflect its Latin etymon.
etymonline
Christianity (n.)
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.
