The term was coined by Prof. Thomas 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. He took it from St. Paul's mention of the altar to ‘the Unknown God.’

-From the etymology for "agnostic" in the OED.

Please note the late date of this event. Many so-called "atheists" in the history of philosophy were actually what we would call "agnostic" today. David Hume is an excellent example.