Christian (n., adj.)

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.