What can we know about God and all other things?

Whatever else Christianity teaches, there is little disagreement that God's creating everything is a kind of rock bottom truth, essential to the Christian position. There is something, therefore, that we can know about everything; we can know that everything is created, except, of course, God ... If everything is created except God, then God must be of an entirely different order than everything else. This mat seem too obvious on the surface, but because of its importance it needs to be said clearly here. It is not the case that everything is created, but rather it is that everything is created except God, who himself is the creator. And we would be quick to add that there was nothing that in any way made or created God. It seems, then, that we have a duality of "existences" in our Christian approach: we have things that exist as created, and One who does not.

( Oliphint, K. Scott. Reasons [for Faith]: Philosophy in the Service of Theology. Phillipsburg, NJ: P&R Pub., 2006. Print. 84-5)

Secular writers and the Spirit of God

John Calvin affirms that grace is evident in knowledge of all sources:

Whenever we come upon these matters in secular writers, let that admirable light of truth shining in them teach us that the mind of man, though fallen and perverted from its wholeness, is nevertheless clothed and ornamented with God's excellent gifts. If we regard the Spirit of God as the sole fountain of truth, we shall neither reject the truth itself, nor despise it wherever it shall appear, unless we wish to dishonor the Spirit of God. (Institutes 2.2. 15)

This should give us a sense of gratitude for the light that radiates from humanity, whether it be sacred or secular.