antichristian adj.

mid-14c., earlier antecrist (late Old English) "an opponent of Christ, an opponent of the Church," especially the last and greatest persecutor of the faith at the end of the world, from Late Latin antichristus, from Greek antikhristos (I John ii.18), from anti- "against" (see anti-) + khristos (see Christ). The earliest appearance of anti- in English and one of the few before c. 1600.

The name has also been applied to the pretenders to the Messiahship, or false Christs (Mat. xxiv. 24), who have arisen at various periods, as being antagonistic to the true Christ. Of these as many as sixty-four have been reckoned, including some of little importance, and also some, as Mohammed, who cannot properly be classed among them. [Century Dictionary]

variant of suffix -an (q.v.), with connective -i-. From Latin -ianus, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective. In Middle English frequently it was -ien, via French.