Given that skepticism is a self-defeating epistemology (see here), one must give an epistemological account for what one thinks he knows. I mentioned the argument from morality a few posts ago, noting that moral nihilists had an adequate response to it. There is an equally short argument, however, which is destructive to all atheists. It might be called the argument from the contingency of knowledge:
For a being to claim to know a proposition is true presupposes that he knows it's truth is not contingent or, if it is, said being knows that upon which the veracity of the proposition is contingent. Let's call this being Ryan. This implies two things:
1. Ryan is omniscient or has acquired his knowledge from a being who is omniscient, as Ryan would be required to know the relation between a proposition and anything upon which the veracity of the proposition might be predicated (which in turn implies knowledge of everything, including respective contingencies).
2. Ryan's knowledge is infinite or has acquired his knowledge from a being whose knowledge is infinite, as there are infinitely many possible relations one might posit between the proposition in question and everything imaginable.
Or we may consider these questions: how does Ryan know the proposition he claims is true isn't contingent on x, y, or z? If Ryan doesn't know, can he justifiably claim to know the proposition is true? No.
What does this mean? A being necessarily exists who is omniscient, infinitely knowledgeable, and has revealed himself to men. While this isn't a unique evidence for Christianity - see the above blog post for that - it is a concise theistic argument that one may find useful.
P.S. Should anyone ask how I know that this argument is self-affirming, I'd point him to the Bible.