Practical Atheism
Nobody is really an atheist because we see in Romans ch.1 that we have been created with an awareness, knowledge that God exists. However, people's sinful hearts supress that knowledge. Yet many who don't do that, who believe and accept that there is a God are practical atheists. The whole of their lives, aims and purposes are carried on as if there is no God. People will regularly say there is a God they believe in, and yet in their conduct live as if there is no God, "God is not in all his thoughts" (Psalm 10 v 4).
Some people acknowledge the existence of God, His being creator, and His government of the world; but live as if God didn't see their behaviour which they are accountable to God for. How foolish to think we can see, hear and know things but God cannot see, hear and know all we are doing (Psalm 94 v 7-11). Do you really suppose that a holy, righteous God will overlook all the ways you act, speak and think that are contrary to His commands? Every aspect of your life comes under the scrutiny of an all-knowing God. Do you really imagine God will not rightly, justly and deservedly punish you for disobeying your maker, who has as such total authority over you? Don't be so foolish to think God will not take any notice of the way you live (Psalm 94 v 7).
It is the fool that says, "No to God", which is what Psalm 53 v 1 literally means. So, don't be foolish, wise up and seek God's forgiveness and a personal relationship with the one true living God by repenting and believing on the Lord Jesus Christ.