WordNet sense 1 (rejecting any belief in gods):

atheistic, atheistical

WordNet sense 2 (hostile or indifferent to religion):

irreligious

WordNet sense 3 (holding that only material phenomena can be known and knowledge of spiritual matters or ultimate causes is impossible):

nescient

WordNet sense 4 (uncertain of all claims to knowledge):

agnostic, agnostical

WordNet sense 5 (denying or questioning the tenets of especially a religion):

disbelieving, skeptical, sceptical

WordNet sense 6 (not disposed or willing to believe; unbelieving):

incredulous