In order for you to know something, it must be true, you must believe it, and you must have some kind of warrant for it.
Believing occurs inside of you, and belief is “about” something. My computer desk cannot be about some other idea. A piece of raw meat cannot be about Hector Berlioz. However a belief can be about Hector Berlioz. “Hector Berlioz wrote thematic music” is a belief about Hector Berlioz. It would appear that desks, meat, and other such natural objects do not share this feature with beliefs (“aboutness”). It appears that beliefs are not reducible to being natural or physical things.
Truth is the same way. Truth has no mass, charge etc.; no scientifically measurable qualities. If you define reality as being composed of nothing other than what is physical, material, natural, whatever; then you have neither belief nor truth available for you to use in your understanding of knowledge. Yet knowledge requires these two things, as I have already mentioned.
You also need to have warrant if you are going to have knowledge. It is not enough to just happen to believe something that turns out to be true; that is not knowledge. For example, if you believe that it is snowing in Miami, and it really is, but you believe it because you had a dream that it is snowing in Miami, then you do not have warrant and also do not have knowledge. If you see that it is snowing in Miami though, then you have warrant. There is a right way and a wrong way to believe things.
The trouble is, a person who does not believe in the Christian God has no basis upon which to say that there is a real “right” or “wrong” to anything. Not only is there no room for belief and truth, but there is not room for a standard of right and wrong ways to come to believe something or to continue to believe something. The concept of beliefs having or lacking warrant is necessary for knowledge, but the concept is inconsistent with what non-Christians want to say about the world.
Christianity allows for these three parts of knowledge without much difficulty.
To have knowledge of something you must believe it, it must be true, and you must have warrant. Non-Christians are left without a basis for any of these three things and these things are inconsistent with non-Christianity. If Christianity is false, then we cannot know anything. Clearly we have knowledge, so Christianity is not false, it is true. If you deny that Christianity is true then you deny that you can have knowledge and so you defeat yourself.