Many Christians use Psalm 14:1 to indicate what God says about atheists. The word for fool is × ָבָל, nabal , which essentially means a vile person. Although many professing atheists display a profound lack of reasoning ability, nabal does not indicate lack of intelligence, but moral failing. But you knew that. There is a big problem with Christians who act like God does not exist. We can effectively be atheists by saying in our hearts that there is no God; Christians can be fools. Phil Johnson has an excellent sermon about this called "Atheism in our Hearts". I hope you will download it (there's an arrow down at the far right of the thing below) or listen online.
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