Agnostic doesn't mean one who has no knowlege of spiritual matters but actually one who believes they are unknown by all and probably unknowable
This from the OED:
A. n. One who holds that the existence of anything beyond and behind material phenomena is unknown and (so far as can be judged) unknowable, and especially that a First Cause and an unseen world are subjects of which we know nothing.
They agree with the Huxley derivation though.
PS The New Testament as a reliable historical source? - Now I've heard everything! :c)