Doubting the Obvious

Quoting K. Scott Oliphant:

God is always and everywhere making his character known. He is successful in every case. Those to whom God reveals himself clearly see and understand what God is revealing. It is that clear understanding that renders them without excuse. If God’s revelation were not clear—if it were obscure or ambiguous—then Paul’s argument would be in error. People would have an excuse if God’s truth given through natural revelation did not get through. But it does get through; it gets through to every person. Whenever we think of defending the Christian faith, we must recognize that whoever our audience is, God has already and always been speaking to them.

Even though God is incomprehensible (in the sense that we will never have exhaustive knowledge of him), we can understand—and do understand—what he reveals of himself through natural revelation (or through the sensus divinitatus).

However, 

because of the sinfulness of man that pervades us in Adam, the sensus [see link above] is the one thing that we will diligently work night and day to doubt, even to deny. But this doubting and denial is just an expression of the suppression of the truth that is attached, like a cancerous tumor, on our Adamic status. In Adam we will, even if it kills us, do all that we can to avoid what is patently and clearly made obvious to us by God himself.

From Covenantal Apologetics: Principles and Practice in Defense of Our Faith.