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.
c. 1200, religioun, "state of life bound by monastic vows," also "action or conduct indicating a belief in a divine power and reverence for and desire to please it," from Anglo-French religiun (11c.), Old French religion, relegion "piety, devotion; religious community," and directly from Latin religionem (nominative religio) "respect for what is sacred, reverence for the gods; conscientiousness, sense of right, moral obligation; fear of the gods; divine service, religious observance; a religion, a faith, a mode of worship, cult; sanctity, holiness," in Late Latin "monastic life" (5c.).
This noun of action was derived by Cicero from relegere "go through again" (in reading or in thought), from re- "again" (see re-) + legere "read" (see lecture (n.)). However, popular etymology among the later ancients (Servius, Lactantius, Augustine) and the interpretation of many modern writers connects it with religare "to bind fast" (see rely), via the notion of "place an obligation on," or "bond between humans and gods." In that case, the re- would be intensive. Another possible origin is religiens "careful," opposite of negligens.
In English, the meaning "particular system of faith in the worship of a divine being or beings" is by c. 1300; the sense of "recognition of and allegiance in manner of life (perceived as justly due) to a higher, unseen power or powers" is from 1530s.
mid-15c., "adherent of a religion opposed to Christianity," from Old French infidèle, from Latin infidelis "unfaithful, not to be trusted," in Late Latin "unbelieving" (in Medieval Latin also as a noun, "unbeliever"), from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (from PIE root *bheidh- "to trust, confide, persuade").
Originally "a non-Christian" (especially a Saracen); later "one who does not believe in religion, disbeliever in religion generally" (1520s). Also used to translate Arabic qafir (see Kaffir), which is from a root meaning "to disbelieve, to deny," strictly referring to all non-Muslims but virtually synonymous with "Christian;" hence, from a Muslim or Jewish point of view, "a Christian" (1530s). As an adjective from mid-15c., "of a religion opposed to Christianity;" 1520s as "rejecting the Christian religion while accepting no other."
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.
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.
el Cid, title applied in Spanish literature to dauntless Castilian nobleman and warlord Ruy Diaz, Count of Bivar (c.1040-1099), champion of the Christian religion and the old Spanish monarchy against the Moors, 1680s, from Spanish cid "chief, commander," from Arabic sayyid "lord."