1. atheistic, atheistical, unbelieving -- (rejecting any belief in gods)
2. atheist, atheistic, atheistical -- (related to or characterized by or given to atheism; "atheist leanings")
1.
"A future life" Prince Andrew repeated, but Pierre, giving him no time to reply, took the repetition for a denial, the more readily as he knew Prince Andrew's former atheistic convictions. - from War and Peace by Leo Tolstoy