In studying Acts 17, I found this paragraph interesting from RC Sproul in his commentary:
"I do not argue with atheists. I simply say, "You know very well that God exists. Your problem isn't that you don't know that God exists; your problem is that you can't stand Him." The sin of man is not atheism; it is religion, which is how we distort God's revelation of himself. We shape a deity in our own image and make houses for Him with our own hands, and serve and worship the creature rather than the Creator; exchanging the glory of God for a life (Rom. 1:25). Paul the told the Romans that God so manifests himself that every human being knows God, and knowing we refuse to worship Him as God, neither are we grateful, and we turn ourselves to idols (Rom. 1:21). That is what religion is---the substitution of a false god for the true God. Even Christianity can become a religion when we substitute the God of scripture for a god of our making."