21 Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
[AD 202] Irenaeus on Romans 1:21
For he judges the Gentiles, "who serve the creature more than the Creator"

[AD 258] Cyprian on Romans 1:21
That idols are not gods, and that the elements are not to be worshipped in the place of gods. In the cxiiith Psalm it is shown that "the idols of the heathen are silver and gold, the work of men's hands. They have a mouth, and speak not; eyes have they, and see not. They have ears, and hear not; neither is there any breath in their mouth. Let those that make them be made like unto them." Also in the Wisdom of Solomon: "They counted all the idols of the nations to be gods, which neither have the use of eyes to see, nor noses to draw breath, nor ears to hear, nor fingers on their hands to handle; and as for their feet, they are slow to go. For man made them, and he that borrowed his own spirit fashioned them; but no man can make a god like unto himself. For, since he is mortal, he worketh a dead thing with wicked hands; for he himself is better than the things which he worshippeth, since he indeed lived once, but they never." In Exodus also: "Thou shalt not make to thee an idol, nor the likeness of anything." Moreover, in Solomon, concerning the elements: "Neither by considering the works did they acknowledge who was the workmaster; but deemed either fire, or wind, or the swift air, or the circle of the stars, or the violent water, or the sun, or the moon, to be gods. On account of whose beauty, if they thought this, let them know how much more beautiful is the Lord than they. Or if they admired their powers and operations, let them understand by them, that He that made these mighty things is mightier than they."

[AD 311] Methodius of Olympus on Romans 1:21
Now certainly the wretched ones were overwhelmed in the chaos of error, "because that, when they knew God, they glorified Him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened; "

When I reflect, O Emperor Constantine, and often revolve in my mind the original condition of men, it is accustomed to appear alike wonderful and unworthy that, by the folly of one age embracing various superstitions, and believing in the existence of many gods, they suddenly arrived at such ignorance of themselves, that the truth being taken away from their eyes, the religion of the true God was not observed, nor the condition of human nature, since men did not seek the chief good in heaven, but on earth. And on this account assuredly the happiness of the ancient ages was changed. For, having left God, the parent and founder of all things, men began to worship the senseless works of their own hands. And what were the effects of this corruption, or what evils it introduced, the subject itself sufficiently declares. For, turning away from the chief good, which is blessed and everlasting on this account, because it cannot be seen, or touched, or comprehended, and from the virtues which are in agreement with that good, and which are equally immortal, gliding down to these corrupt and frail gods, and devoting themselves to those things by which the body only is adorned, and nourished, and delighted, they sought eternal death for themselves, together with their gods and goods relating to the body, because all bodies are subject to death.

[AD 360] Eusebius of Emesa on Romans 1:21
It may seem that Paul has simplified his argument against the Greeks, because he condemns idolatry as the only kind of ungodliness. But to those who look more carefully at what he says, things will appear not this way but rather that Paul has broadened his horizons, so as not to overlook any kind of impiety.

[AD 382] Apollinaris of Laodicea on Romans 1:21
It is futile to conceive of nonbeing as if it were being, as do those who worship vanity as if it were God. For this reason their minds have been rendered senseless and darkness has entered their souls.

[AD 384] Ambrosiaster on Romans 1:21
They were so far from being ignorant that they confessed that there was a single principle from which all things heavenly, earthly and infernal derived their origin, and that there was only one being who decreed what properties and duties would belong to everything by nature. Yet knowing this they did not give thanks. Paul is speaking of the ancients in order to correct his contemporaries and future generations.Truly this is futility, that knowing the truth they decided to worship something else which they knew was not true, so that hiding from God they might worship idols. A cloud of error covered their hearts. Although they should have honored the Creator all the more from the beautiful things which he made, they clung to what they had, saying that the things which they could see were sufficient for their salvation.

[AD 407] John Chrysostom on Romans 1:21
This is the one greatest charge; and the second after it is their also worshipping idols, as Jeremy too in accusing them said, "This people hath committed two evils: they have forsaken me the fountain of living water, and have dug for themselves broken cisterns." (Jer. ii. 13.) And then as a sign of their having known God, and not used their knowledge upon a fit object, he adduces this very thing, that they knew gods. Wherefore he adds, "because that, when they knew God, they glorified Him not as God." And he names the cause through which they fell into such senselessness. What then is it? They trusted everything to their reasonings. Still he does not word it so, but in a much sharper language, "but became vain in their reasonings, and their foolish heart was darkened." For as in a night without a moon, if any one attempt to go by a strange road, or to sail over a strange sea, so far will he be from soon reaching his destination, that he will speedily be lost. Thus they, attempting to go the way leading to Heaven, and having destroyed the light from their own selves, and, in lieu of it, trusted themselves to the darkness of their own reasoning, and seeking in bodies for Him who is incorporeal, and in shapes for Him who hath no shape, underwent a most rueful shipwreck. But beside what has been said, he names also another cause of their error, when he says,

[AD 418] Pelagius on Romans 1:21
Imagining that they could grasp God with their minds, they fell away from their natural instinct and worshiped creatures instead of the Creator.

[AD 430] Augustine of Hippo on Romans 1:21
If they had given thanks to God who gave this wisdom, they would not have claimed any credit for their own ideas. Therefore they were given over by the Lord to the desires of their own hearts, and did improper things.

[AD 430] Augustine of Hippo on Romans 1:21
Surely that darkening of the heart was already a penalty and punishment, and by that penalty, that is by the blinding of the heart because of the abandonment by the light of wisdom, they fell into more and more serious sins.

[AD 455] Prosper of Aquitaine on Romans 1:21
It is well known how Greek schools and Roman eloquence and the search of the whole world in the quest of the supreme good, with the most penetrating study and outstanding ability, accomplished nothing by their labor except to become “futile in their thinking, and their senseless minds were darkened.”

[AD 471] Gennadius of Constantinople on Romans 1:21
The pagans knew that there was a God, and it is clear that they did not receive judgment because of this. For it was not for want of knowledge that they were condemned, but the opposite. For each one glorified God in the sense that whatever he thought God was, that he served. Thus they corrupted the whole matter by their peculiar and mistaken ideas. They abandoned God’s way of knowing him and preferred their own, falling into the deepest imbecility, outdoing themselves in their so-called wisdom by adding to their foolishness, descending to the worship of reptiles and inanimate objects.

[AD 542] Caesarius of Arles on Romans 1:21
In the hearts of the Gentiles, the purest honoring of the one God was changed into the bloody worship of different gods.