For what can be known about God is plain to them, because God has shown it to them. (ESV)
There are those who claim to be atheists. An atheist claims that it is a provable fact that there is no god. That is a bold claim. In this verse God makes a bold claim. He is essentially saying that he does not believe in atheists! Why does God not believe in atheists? Because he has shown all men that he exists. What can be known about God is plain to them.
In the previous verse Paul laid out the point that it is men's unrighteousness that suppresses the truth. It is not that men do not know that God exists, it is that they take that knowledge and suppress it with their ungodly behavior. They would rather walk in darkness than allow the truth of God to penetrate their lives.
One of the greatest philosophers of the twentieth century was a man called C. S. Lewis. Lewis was a professor at both Oxford and Cambridge. He became one of the greatest defenders of Christianity. In his earlier years he claimed to be an atheist. It is interesting how he describes the conundrum of calling himself an atheist:
“My argument against God was that the universe seemed so cruel and unjust. But how had I got this idea of just and unjust? A man does not call a line crooked unless he has some idea of a straight line. What was I comparing this universe with when I called it unjust?”
― C.S. Lewis, Mere Christianity
There is a moral law that is written on the heart of every man. This is part of what God has made plain to man that reveals God's existence. That reality was one of the key turning points in Lewis coming to faith. All of mankind can know God. God has made himself known. The question for man to wrestle with is how will he respond to the knowledge he has received.
How do you respond to the knowledge of God you have received? Do you submit to him as Lord? Do you defy his existence and suppress the truth by your own unrighteousness? Yield to him. He is a loving Lord and wants you to know him more.