Lewis & Short

Christĭānus, a, um, adj. [Christus], Christian: fides, Cod. Just. 16, 8, 18: lex, ib. 16, 8, 13: religio, ib. 9, 40, 16.
Hence, subst., a Christian, Tac. A. 15, 44; Suet. Ner. 16; Plin. Ep. 10, 97; very frequent in the Church fathers.
Absol., a Christian clergyman, Cod. Th. 5, 5, 2; 12, 1, 50.
Sup.: Christianissimus, the most Christian, Hier. Ep. 57, 12: princeps, Ambros. Ep. 1, 1.
Adv.: Christĭānē, in a Christian manner or spirit: regere, Aug. Ep. 89.