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.
c. 1600, "one engaged in war or strife," from militant (adj.); in the political sense of "one seeking change by use or advocacy of direct action" it is attested by 1909.
early 14c., "member of a militant 1st century Jewish sect which fiercely resisted the Romans in Palestine," from Late Latin zelotes, from Greek zēlōtēs "one who is a zealous follower," from zēloōn "to be zealous," from zēlos "zeal" (see zeal). Extended sense of "a fanatical enthusiast" first recorded 1630s (earlier in this sense was zelator, mid-15c.).
1881, from opportunism (q.v.) + -ist. A word in Italian politics, later in France opportuniste was applied derisively to the moderate Léon Gambetta (1876), leader of the party between the monarchists and the extreme republicans. In English the word was used generally of anyone whose policy or tendency is to seek to profit from the prevailing circumstances or take advantage of opportunities as they occur.
Once seated in the legislature Gambetta argued that all republicans—the old guard, young republicans, and even recent converts—could and should cooperate. He preached compromise and accommodation—Opportunism—in order to achieve the politically possible. He spoke against violent revolution and sought to promote peaceful reforms using legal methods, a stance that pitted him directly against the militant demagogue Henri Rochefort, who latched onto the term Opportunism as a term of abuse. [Robert Lynn Fuller, "The Origins of the French Nationalist Movement, 1886-1914"]