The reason a theist can never win a debate against an atheist is quiet simple actually
in Religion

Burden of proof (also known as onus probandi in Latin) is the obligation on somebody presenting a new idea (a claim) to provide evidence to support its truth (a warrant). Once evidence has been presented, it is up to any opposing "side" to prove the evidence presented is not adequate. Burdens of proof are key to having logically valid statements: if claims were accepted without warrants, then every claim could simultaneously be claimed to be true.