In the final paragraph of Rene Descartes' first Meditation, he introduces the idea of a demon who is "supremely powerful and clever, who has directed his entire effort at deceiving [him]," and it has occurred to me that, without a knowledge of the gospel, a person would be unable to prove that no such demon exists.
One would think that God would never allow such a demon to get away with that, but one could be deceived about both the existence of the demon and the existence of God. If such a demon were messing with our heads, how would we know it?
Epistemologically speaking, I don't have a good answer for this, but in the light of the gospel, I do. I have a strong enough testimony to know that God exists and that He is good enough to not let any demon so thoroughly deceive us.
Once again, a great challenge of philosophy is solved easily by religion. The rest of the world may be stuck with nagging doubts that they can't be sure that they aren't being deceived, but we know that we can trust our perceptions because we know that God wouldn't let any demon deceive us like that.