My favourite exponent of non-theism has for a long time been A.J. Ayer, and in fact the other positivists, due to his simple statement of non-theism: To ask whether God exists is simply not meaningful.

In this context it means that no-one can produce evidence one way or the other as to God's existence; there is no test, positive or negative. You cannot formulate a model of the world with God, and the world without God, and then compare the universe as it is to your models. The problem inheres on at least two levels: First, God is seen as being outside the universe; second, you only have one universe.

Concerning anything outside the universe, one can have no statements, for the universe is the entirety of what we can experience, and hence know. The only way god could conceivably "exist outside the universe" is if it were some sort of mathematical theorem. As we only have one universe, we cannot speculate what universes with or without God would be like, and hence no test can be made; as such, to say "God exists", or "God does not exist" is the same as saying "Green is green-coloured".