Psalm 53 - Atheists are Fools.

Psalm 53 - You are a Fool if you Don't Believe in God.

 The fool says in his heart, “There is no God.” They are corrupt, doing abominable iniquity; there is none who does good.

"The fool hath said in his heart, There is no God." And this he does because he is a fool. Being a fool he speaks according to his nature; being a great fool he meddles with a great subject, and comes to a wild conclusion. The atheist is, morally, as well as mentally, a fool, a fool in the heart as well as in the head; a fool in morals as well as in philosophy.   http://biblehub.com/commentaries/tod/psalms/53.htm

A fool is: idiot, ass, blockhead, dunce, dolt, ignoramus, imbecile, cretin, dullard, simpleton, moron, clod;
Well, Jesus said we are not to call people fools because we don't know people and have no right to judge. However, here God calls an atheist a fool. He alone has the right to judge others.

 God looks down from heaven
on all mankind
to see if there are any who understand,
any who seek God.

Everyone has turned away, all have become corrupt;
there is no one who does good,
not even one.  Verses 2,3

All mankind has sinned. We are all born with a stone heart. Our first birth into this world of sin and sadness is flawed. But thank God, he has given us a new birth where we may have a new and softer heart. We won't be perfect, but he will slowly change us so our heart beats along with his great heart and we will love what he loves and hate sin.