Atheism (noun):
Disbelief in, or denial of, the existence of a God. Also, Disregard of duty to God, godlessness (practical atheism).
Agnosticism (noun):
The doctrine or tenets of agnostics with regard to the existence of anything beyond and behind material phenomena or to knowledge of a First Cause or God; relating to the belief that the existence of anything beyond and behind material phenomena is unknown and (as far as can be judged) unknowable.
Humanism (noun):
A variety of ethical theory and practice characterized by a stress on human rationality and capacity for free thought and moral action, and a rejection of theistic religion and the supernatural in favor of secular and naturalistic views of humanity and the universe.