John Calvin Commentary Romans 1:23

"and changed the glory of the incorruptible God for the likeness of an image of corruptible man, and of birds, and four-footed beasts, and creeping things." — Romans 1:23 (ASV)

And changed, etc. Having feigned such a God as they could comprehend according to their carnal reason, they were very far from acknowledging the true God, but devised a fictitious and a new god, or rather a phantom. And what he says is that they changed the glory of God; for just as one might substitute a strange child, so they departed from the true God. Nor are they to be excused for this pretense, that they believe God dwells in heaven, and that they do not consider the wood to be God, but His image; for it is a high indignity to God to form so crude an idea of His majesty as to dare to make an image of Him. But from the wickedness of such a presumption, none were exempt: neither priests, nor statesmen, nor philosophers, of whom the most sound-minded, even Plato himself, sought to find some likeness of God.

The madness then noted here is that all attempted to make for themselves an image of God, which was a clear proof that their notions of God were crude and absurd. And, first, they befouled the majesty of God by forming Him in the likeness of a corruptible man: for I prefer this rendering to that of mortal man, which is adopted by Erasmus; for Paul does not set the immortality of God in opposition to the mortality of man, but rather that glory, which is subject to no defects, to the most wretched condition of man. And then, not being satisfied with so great a crime, they descended even to beasts and to those of the most filthy kind, by which their stupidity appeared even more evident. You may see an account of these abominations in Lactantius, in Eusebius, and in Augustine in his book On the City of God.