Psalm 53:1 Explained: The Fool Says In His Heart, "There Is No God"
"The fool says in his heart, 'There is no God.'" (Psalm 53:1).
When the Holy Spirit says this, He's not directly saying that every professing atheist is a fool. Rather, he is saying that one of the trademark behaviors of the fool is professing atheism.
Why do fools adopt atheism? The passage gives us the answer: These people oppose God, and God opposes them. They believe that, by denying God is there, they will be free to pursue corruption, persecute God's people, and ignore God entirely.
"They are corrupt, doing abominable iniquity; there is none who does good."
The Psalm goes on, and ends up categorizing all the unbelieving world as "fallen away," "corrupt," and failing to do good. These ones are contrasted with those the author calls "my people" and whom God refers to as his people, or "Jacob" and "Israel" (in the New Covenant era, this refers to followers of the Messiah Jesus).
In other words, while God is not directly saying, "Every self-proclaimed atheist is a fool," He does seem to be saying that there are only two ways to live: as God's people, or as fools. And every fool says in his heart (even if he doesn't vocally profess it), that "There is no God."
Here's a cool, ancient English translation of this Psalm, from the Geneva Bible, if you want to read it in context.