early 15c., "fighting, engaged in warfare," from Old French militant "fighting" and directly from Latin militantem (nominative militans), present participle of militare "serve as a soldier" (see militate). The sense of "having a combative character or tendency," especially "seeking political change by use or advocacy of direct action," is by 1907. Related: Militantly.
Originally especially in Church militant (early 15c., chirche militans), which is the Church on earth, seen as engaged in warfare with the devil, the flesh, and worldly powers of temptation and unrighteousness. The Church triumphant (1550s) is the collective body of saints now glorified in heaven.