It is not a matter of them thinking God does not exist, but that they are denying Him. They do not want to believe that He is there. He has denied God by denying or renouncing what God says about Himself.

God says plainly in many scriptures that He will judge for sin. This is what the Ten Commandments are about: They define sin for us. By them, we know what is good and what is not good, so we can come into judgment before Him without excuse. A reason Christ came was to qualify to be that Judge, to judge all at the last day. We, however, are going through our judgment right now.

The wicked man has fooled himself into believing that God will not require an accounting for sin. He thinks God will not pass judgment on him; He will just let his sins slide.

It sounds much like modern, antinomian Protestantism. It is, in effect, what the psalmist is talking about. He speaks of a normal, everyday Israelite who would supposedly know God. He had entered into the covenant by circumcision, and so he should know better. However, he has decided to ignore what God has said and lives his life as a practicing atheist. He is not really an atheist, because he believes there is a God, but he lives his life as if God did not exist.

The wicked man here, then, could be somebody who professes Christ—a professing Christian—but his lifestyle betrays his real, inner conclusion that there is no God.