Mathematical proofs need no physical evidence. They're saying something similar is accepted by some Christians. That's an internally consistent viewpoint. Us skeptics simply misunderstand their irrefutable logical proof.

Well, there are some atheists that argue that God's omnibenevolence is incompatible with the evils in the world, meaning God cannot exist (or at least, is either not omnibenevolent or not omnipotent, and if these are part of the definition of God, then God insofar as the term refers to something with at least these two properties, does not exist). Theists have responses to this argument, but the point is that the subject matter is something that can be rationally discussed.