I try to be careful never to call another person a fool as God has warned us not to do that. So I am only repeating what God has said. God has called some people fools. He said if a person does not believe in God he is a fool. I would have to say that there are various things people do or say that qualifies them as a fool. Yes, God has said anyone who believes there is no God is a fool. Psalm 14:1

“Both Psalm 14:1 and Psalm 53:1 read, “The fool says in his heart, ‘There is no God.’” Some take these verses to mean that atheists are stupid, i.e., lacking intelligence. However, that is not the only meaning of the Hebrew word translated “fool.” In this text, the Hebrew word is nabal, which often refers to an impious person who has no perception of ethical or religious truth. The meaning of the text is not “unintelligent people do not believe in God.” Rather, the meaning of the text is “sinful people do not believe in God.” In other words, it is a wicked thing to deny God, and a denial of God is often accompanied by a wicked lifestyle. The verse goes on to list some other characteristics of the irreligious: “They are corrupt; their deeds are vile; / there is no one who does good.” Psalm 14 is a study on the universal depravity of mankind.”… https://www.gotquestions.org/fool-heart-no-God.html

A lot of people believe in God, but what God do they believe in? There is only one God and He is the God of the bible. If a person believes wrongly about God, no matter how sincere they are, they are sincerely wrong. And we are without excuse as God has given us His Word. And He commands us to study it, not just read it. We are to study it so that we properly understand it and come to know the God of the bible and please Him. Of course, that involves obeying God as well.

I have met a lot of people who readily say, Oh, I believe in God. However, it is quickly obvious that they don’t know Him. And He doesn’t know them. By that I mean that there is no personal relationship with Him. Without a personal relationship they are lost in their sin and hell bound. That is never God’s will for anyone. He wants every one to turn to Him in repentance. He awaits each and every one of us. He has already done His part. Now it is our turn to do our part. Call upon the Name of Jesus while He may be found. If you end up in hell it will be your fault not God’s. And if you are going to be a fool, be a fool for Jesus … something you will never regret.