One thing to note. God can and does call people ‘fools’ specifically. That is His prerogative, since He is God, and fully knows each human heart.
For example, Jesus told the parable of the rich man whose barns overflowed with all his produce. So he built new, bigger barns that could hold his abundance of wealth. God’s reaction was, ‘You fool, this night your life will be demanded of you; and the things you have prepared, to whom will they belong?’
Bottom line. We are prohibited from calling each other fools. Our knowledge of others’ hearts is too limited. God is free to do so, and does; His knowledge of hearts is unlimited.
God says an unbeliever is a fool. You don’t have to know a man’s heart to know he is an unbeliever. The atheist clearly declares he does not believe in God; therefore, in accordance with God’s Word, he can be labeled a “fool.” “The fool has said in his heart, there is no God.” Therefore, if you declare yourself to be an unbeliever, I am a liberty to quote the Word of God and define you as a “fool.” God is the one who says you are a fool, I’m simply agreeing with His Word, and have the boldness to speak it.