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There's negative atheism which, in a nutshell is a denial of the belief in god with regards to the various arguments the theists put forward; that those arguments are demonstrably false.
Positive atheism goes further and asserts that god is ontologically impossible; the structure of reality precludes the possibility of omniscience, omnibenevolence, omnipotence, perfect foreknowledge and the like. These concepts are shown to be incoherent via reductio arguments. The current most capable defender, imo, of the theist argument [it's an ontological argument] is Billy Joe Lucas. Patrick Grim and others have used Cantorian and Godelian arguments to show that the idea of omniscience is incoherent.