How one defines atheism is typically a function of their existing belief system.
Secular
- The narrowest definition of atheism is the “lack of belief” in a god or a supernatural entity.
- A more common definition of atheism is the “belief” that a god or a supernatural entity does not exist. (This, however, is just a logical conclusion of the narrowest definition.
Religious
- The broadest definition of atheism is anyone who does not believe in “your particular” version of god. In the case of Christians, atheists are people who don’t believe in the Christian God. In the case of true/fundamental Christians, atheists are people who don’t believe in their strict interpretation of the Christian God. By this broad definition (Catholic/Protestant) Christians, Jews, (Sunni/Shiite) Muslims, Hindus, and non believers in general are all atheists in each other’s eyes.
The more religious you are the broader your definition of atheism will be. The less religious you are the narrower your definition of atheism will be.