1870, "one who professes that the existence of a First Cause and the essential nature of things are not and cannot be known" [Klein]; coined by T.H. Huxley, supposedly in September 1869, from Greek agnostos "unknown, unknowable," from a- "not" (see a- (3)) + gnōstos "(to be) known" (from PIE root *gno- "to know"). The coinage is sometimes said to be a reference to Paul's mention of the altar to "the Unknown God" in Acts, but according to Huxley it was a reference to the early Church movement known as Gnosticism (see Gnostic). The adjective also is from 1870.
I ... invented what I conceived to be the appropriate title of 'agnostic,' ... antithetic to the 'Gnostic' of Church history who professed to know so much about the very things of which I was ignorant. [T.H. Huxley, "Science and Christian Tradition," 1889]
The agnostic does not simply say, "I do not know." He goes another step, and he says, with great emphasis, that you do not know. [Robert G. Ingersoll, "Reply to Dr. Lyman Abbott," 1890]