1570s, "Christianity, the Christian religion," from Christian + -ism. Obsolete, but revived or recoined c. 2004 in reference to politicized fundamentalist Christianity in the U.S. Related: Christianist.
1520s as a noun, "a believer in and follower of Christ;" 1550s as an adjective, "professing the Christian religion, received into the Christian church," 16c. forms replacing Middle English Cristen (adjective and noun), from Old English cristen, from a West Germanic borrowing of Church Latin christianus, from Ecclesiastical Greek christianos, from Christos (see Christ). First used in Antioch, according to Acts xi.25-26:
And when he had found him, he brought him unto Antioch. And it came to pass, that a whole year they assembled themselves with the church, and taught much people. And the disciples were called Christians first in Antioch.
Meaning "having the manner and spiritual character proper to a follower of Christ" is from 1590s (continuing a sense in the Middle English word). Christian name, that given at christening, is from 1540s (also continuing a sense from Middle English Cristen). Christian Science as the name of a religious sect is from 1863.
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
<a href="https://www.etymonline.com/word/Christianism">Etymology of Christianism by etymonline</a>
Harper, D. (n.d.). Etymology of Christianism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Christianism