Let’s think of a few clear examples of atheists:
– Richard Dawkins
– David Hume
– football running back Arian Foster
– Sir Richard Branson, a wealthy businessman
– a teenager who reads Camus
What do these examples have in common?
Some are wealthy and famous, others are not. Some defend their atheism with elaborate arguments, others find such arguments unnecessary. Some are swayed by reason or common sense, and others are, perhaps, swayed by aesthetics. They live in different time periods and countries, they have different worldviews, they take atheism to have different implications, and they act differently in real life.
What they all have in common is a grasp of the concept of God and a conscious rejection of that concept. If they did not meet both of these criteria, we would not call them atheists. There would be no surprise or outrage if Richard Dawkins simply had not heard of God, or if Arian Foster had announced that he didn’t know what God was. The teenager reading Camus would not have felt edgy if he simply lacked an understanding of theism. We attach a lot of significance to atheism that we don’t attach to ignorance of or inability to grasp the concept of God.
So, I think it’s pretty clear that atheism should refer to the belief that no God exists, not to a simple absence of belief in God in a person.