Various people use terms such as "atheist" and "agnostic" differently hence they are often difficult to define. Atheism usually means not to believe in God. This can be a mere lack of belief. It can also be a positive belief that He does not exist. Agnostic usually means an individual is not sure whether God exists or not. Perhaps because of the historical risk of being accused of heresy, apostasy or other theological "crimes", those who do not believe in God have not been eager to clarify their positions.