Sometimes atheist assert that there is no proof God exists. The only problem is that an atheist cannot logically make that claim.
In order to state there is no proof for God's existence, the atheist would have to know all alleged proofs that exist in order to then state that there is no proof for God's existence. But, since he cannot know all things, he cannot logically state there is no proof for God's existence.
At best, an atheist can only state that of all the alleged proofs he has seen thus far, none have worked. He could even say that he believes there are no proofs for God's existence. But then, this means there is the possibility that there is a proof or proofs out there and that he simply has not yet encountered one.
Nevertheless, if there was a proof that truly did prove God's existence, would the atheist be able to accept it, given that his presuppositions are in opposition to the existence of God? In other words, given that the atheist has a presuppositional base that there is no God, in order for him to accept a proof for God's existence, he would have to change his presuppositional base. This is not easy to do and would involve a major paradigm shift in the belief structure of the atheist. Therefore, an atheist is presuppositionally hostile to any proofs for God's existence and is less likely to be objective about such attempted proofs.