Epistemologically, if we begin with the assumption that God may or may not exist, then it is impossible to reasonably infer either way. This is the case even with things seemingly fundamental as arithmetic and geometry. In Calvinism, our knowledge of God is innate and a priori. We are not born with a 'tabula rasa', or a blank-mind, which begins to learn through the medium of the senses. It is fascinating to see how many Christians are empiricists. An epistemology must be self-verifying, or circular, in order to be consistent, just as logic must be logical. In this way, if the Christian's epistemology leads him to believe that his belief in God comes from the Holy Spirit, then it would be inconsistent for him to believe otherwise. For if he did believe otherwise, he would be believing something according to a different epistemology, and therefore not thinking as a Christian. Why should he think in accordance to non-christian presuppositions? To think that our belief in God, may not come from God, is to assume non-christian presuppositions. How then do we choose which presuppositions, or first-principles to adhere to? It is logically impossible to be certain, unless there is a God, and God chooses for us. The Christian, or at least the Calvinist, maintains that the atheist already has innate knowledge of God, he is just in rebellion against his creator, and is self-deceived to believe that God does not exist.
Knowing one thing, yet believing it's negation, is the pinnacle of irrational thought.