Echoing the message of Psalm 14, this psalm proclaims the foolishness of atheism (see also Rom_3:10). People may say there is no God in order to cover up their sin, to have an excuse to continue in sin, and/or to ignore the Judge in order to avoid the judgment. "Fools" do not necessarily lack intelligence; many atheists and unbelievers are highly educated. Fools are people who reject God, the only one who can save them.
Psalms 53:4-5
While God is not affected by what we think of him, we are definitely and eternally affected by what God thinks of us. This psalm begins with the bold claim that there is no God, but by this verse, the true reason for rejecting God has become clear. The reason people reject God has nothing to do with God's existence and everything to do with people's sinfulness. In our desire to do wrong, we treat God as if he doesn't exist. When God passes judgment, it will be too late to apologize and admit we were wrong. Rejection of God will turn into terror of God.
