Calvin states that there are three types of reason:
Three kinds of reason are to be considered, but he [Hesushius] at one bound overleaps them all. There is a reason naturally implanted which cannot be condemned without insult to God, but it has limits which it cannot overstep without being immediately lost. Of this we have a sad proof in the fall of Adam. There is another kind of reason which is vicious, especially in a corrupt nature, and is manifested when mortal man, instead of receiving divine things with reverence, would subject them to his own judgment. This reason is mental intoxication, or pleasing insanity, and is at eternal variance with the obedience of faith, since we must become fools in ourselves before we can begin to be wise unto God. In regard to heavenly mysteries, therefore, we must abjure this reason, which is nothing better than mere fatuity, and if accompanied with arrogance, grows to the height of madness. But there is a third kind of reason, which both the Spirit of God and Scripture sanction…
So we have the natural knowledge which is limited and insufficient due to Adam’s fall.
Depraved reason is crazy. It is at variance with “the obedience of faith” (oops!).
There is a third kind of wisdom, which the regenerate possess and use in submission to God’s word.
In mentioning the role of this third reason in the faithful, Calvin states:
We say with Augustine, that when a manifest absurdity occurs, there is a trope or figure in the expression. He answers, that in the judgment of reason nothing is more absurd than that there are three hypostases in the one essence of God, and yet no remedy of a trope is required; as if it were our intention, or had been that of Augustine, to measure absurdity by our carnal sense. On the contrary, we declare that we reverently embrace what human reason repudiates. We only shun absurdities abhorrent to piety and faith.
So the absurdities that are a problem for the faithful are not those that merely seem paradoxical to our carnal sense, but rather the ones that strike at the heart of piety and faith. Ubiquity of the human nature of Christ is one such absurdity, for it undoes orthodox Christology.
This such absurdity is no glory, but rather an attack on the faith itself.