Atheism doesn't simply deny God, it denies one of his or her own existence. This is because what makes one a person is the spiritual soul. Without God there is no spiritual soul, no true self. All that exists is the body and that's all you really are.
But even the atheist tacitly believes in the soul!
Here's an example. An atheist goes to the doctor and is told he has cancer. Upset by the news, the atheist begins his radiation treatment. Of course, he tells his family and friends the news - though he doesn't ask for prayers!
Another atheist goes in for a standard psychological evaluation as part of his job. He is told that he has a mental illness. His reaction: "Don't tell anyone! I don't want people to think there is something wrong with me!"
In other words, the atheist with cancer is not embarrassed by the fact that there is something wrong with his body but the atheist with a mental problem is concerned because there could be something wrong with his soul!
Before I go, Let me make one qualification: someone can have psychological problems which are partially caused by chemical imbalances in the brain. Obviously this would be a bodily problem as well as a potential spiritual problem. The fact is, when people think of psychological issues, they realize deep down that there is something more real and more close to the heart of himself. When an atheist think like this, he is thinking of his spiritual soul and not simply his body.