The point of “The fool says in his heart, ‘There is no God’” is that a lack of evidence of His existence is not the true reason people reject belief in God. People reject belief in God due to a desire to live free of the moral constraints He requires and to escape the guilt that accompanies the violation of those constraints. “The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, since what may be known about God is plain to them … men are without excuse … their thinking became futile and their foolish hearts were darkened. Although they claimed to be wise, they became fools … Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. They exchanged the truth of God for a lie” (Romans 1:18-25).
Read more: http://www.gotquestions.org/fool-heart-no-God.html