agnostic (n.)
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 (1825-1895), supposedly in September 1869, from Greek agnostos "unknown, unknowable," from a- "not" + gnostos "(to be) known" (see gnostic). Sometimes said to be a reference to Paul's mention of the altar to "the Unknown God," but according to Huxley it was coined with reference to the early Church movement known as Gnosticism (see Gnostic).
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 adjective is first recorded 1870.