The word agnostic was coined in 1869 by Thomas Henry Huxley, the noted English biologist. Though the date of coinage is known, the specific etymology that Huxley had in mind has been a matter of debate for some years. The following portion of a letter dated March 13, 1881 (which has since disappeared) from R.H. Hutton was printed by the editors of the Oxford English Dictionary at the entry for agnostic:

Suggested by Prof. Huxley at a party held previous to the formation of the now defunct Metaphysical Society, at Mr. James Knowles’s house on Clapham Common, one evening in 1869, in my hearing. He took it from St. Paul’s mention of the altar to "the Unknown God."

The Greek form of the altar inscription given in Acts 17:23 is agnosto theo.

The Metaphysical Society held an organizational meeting on April 21, 1869, and the first appearance of agnostic in print is in the Spectator for May 29 of that year in an article probably written by the same R.H. Hutton, who was the magazine’s literary editor at the time:

All these considerations, and the great controversies which suggest them, are in the highest degree cultivating, and will be admitted to be so even by those Agnostics who think them profitless of any result.

In 1889, Huxley himself said that he “invented what I conceived to be the appropriate title of ‘agnostic.’”

It came into my head as suggestively antithetic to the "gnostic" of church history, who professed to know so much about the very things of which I was ignorant; and I took the earliest opportunity of parading it at our Society.

The most reasonable statement that can be made reconciling these differing opinions of Hutton and Huxley is that both are correct to a degree and each point of view is influenced by the other.

From The Merriam-Webster New Book of Word Histories.