The above atheist is obviously quite ignorant about the difference between vicious circularity and simply being consistent. Both are circular reasoning. However as Van Til pointed out, only the Theist's worldview (in the classical sense which would include Divine Revelation) a) does not self-destruct (internally inconsistent with itself), b) is not destructive of epistemology (it destroys the possibility of knowledge), or c) is not arbitrary (based on the universalizing of a particular experience or speculative metaphysics).

Atheism usually fails (a) when it is in the non-nihilist form since universal and immutable ethical laws are only possible with a omnipresent, omnipotent, and immutable Being who can reward or punish all men according to their deeds. (Indeed, all non-Theistic worldviews fail this test.)

Atheism fails (b) since it places the origin of man's cognitive faculties in an impersonal, non-teleological, stochastic process of neo-Darwinian evolution which means that man's faculties are survival oriented and not necessarily truth oriented. In fact, the degree to which they are truth oriented is probably zero or inscrutable at best. This would undermine or destroy the possibility of warranted true belief. (Indeed, all non-Theistic worldviews fail this test since they place the origin of mind in a non-teleological, impersonal universe.)

Atheism fails (c) since one cannot logically derive a universal from a particular experience, and one has no epistemic warrant for believing in a universal based on philosophical speculation. In order to have epistemic warrant to believe in a certain universal, one must possess complete knowledge of the universe and how all particulars of the universe relate to each other. For this to be the case, one must be omniscient OR have this universal revealed to him by an omniscient Being. This necessitates propositional Divine Revelation. (Of course, all non-Theistic worldviews fail this test since they lack propositional Divine Revelation.)

Thus, the greatest proof that God exists is that without Him, everything would be absurd.