Nouns

(n)agnostic, doubtersomeone who is doubtful or noncommittal about something
(n)agnostica person who claims that they cannot have true knowledge about the existence of God (but does not deny that God might exist)

Adjectives

(a)agnosticof or pertaining to an agnostic or agnosticism
(a)agnostic, agnosticaluncertain of all claims to knowledge