Pass it on
If the infinite-personal God does not exist, you have a problem that philosophy has spent three thousand years failing to solve: how do you account for the fact that you know anything at all?
The categories you use to think; causality, unity, diversity, being, truth; did not come with the hardware. They are not self-explanatory. You cannot prove the law of non-contradiction without using the law of non-contradiction, which is circular. You cannot establish the reliability of your senses using only your senses, which is circular. Every system of knowledge rests on assumptions it cannot justify from within its own resources. This is not a problem unique to Christianity. It is a problem for every worldview that tries to stand on its own feet.
He Is There and He Is Not Silent
Schaeffer’s argument; compressed into the title of his most famous work; is that the only adequate ground for knowledge is a God who is both there (objectively real, independent of human consciousness) and not silent (has communicated, making knowledge possible). Without this foundation, knowledge becomes either a brute fact with no explanation; "it just works, stop asking"; or a human construction with no anchor; "we made it up, but it’s useful." Neither is satisfying. Neither can account for why science works, why logic compels, why moral categories feel binding rather than optional.
The Christian claim is not that non-Christians cannot know things. They clearly can. The claim is that their knowing is parasitic on a foundation their worldview cannot justify. They are living in God’s universe, using God’s categories, thinking God’s thoughts after Him; and attributing the whole enterprise to random processes that have no interest in truth. The fact that knowledge works at all is evidence that the universe was designed for knowers. And the Knower who designed it is the one who spoke it into existence and continues to hold it together.