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)) + gnostos "(to be) known," from PIE root *gno- "to know." 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 coined with reference to the early Church movement known as Gnosticism (see Gnostic). The adjective also is first recorded 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]