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.