Professing atheists are fools. “The fool says in his heart, ‘There is no God'” (Psalm 14:1). There are also practical atheists, those who, whether God exists or not, live like He doesn’t. Our lives, more than our lips, reflect what we believe in our hearts.
Idolators are also fools. There’s no wisdom in saying that there is no god, there’s also no wisdom in making up a god. And this is key for professing believers: there are practical idolators, too, those who live like the true God is different than He really is. We are image-bearers, not merely image-describers, and, in particular, we are Christians, so that means our behavior should reflect what He looks like.
If I describe God as a personal God but I always keep others at a distance, I reflect an idol, a distant God. That makes me a fool. If I say God is a God of love but I default to criticism or am quick to anger, I reflect an idol, a demanding God. That is practical idolatry; that is foolishness.
It’s easy for us to cry “Fool!” at someone who believes in evolution or someone who bows before a carved block of wood. It is harder to see and confess our own practical foolishness. We believe in a personal and loving God, and we must repent when we do not reflect Him truly.
 I say “professing” atheists because, according to Romans 1:18-21 every man knows God, even if he claims not to.