Absence of belief that any deities exist (including absence of the concept of deities).
Absence of belief in a particular deity, pantheon, or religious doctrine (notwithstanding belief in other deities).
Rejection of belief that any deities exist (with or without a belief that no deities exist).
Belief that no deities exist (sometimes including rejection of other religious beliefs).
Something that is a certainty.
Sureness, certainty.