Because, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish hearts were darkened.
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Ambrosiaster
AD 400
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. Commentary on Paul’s Epistles.
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.
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.
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. .
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 socalled wisdom by adding to their foolishness, descending to the worship of reptiles and inanimate objects. .
This is the greatest charge against the heathen, and the second is the fact that they worshiped idols. … The heathen tried to get to heaven, but having destroyed the light that was in them, instead entrusted themselves to the darkness of their own reasoning. They looked for the incorporeal in bodies and for the infinite in creaturely shapes, and so lost their congruity with the light.
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 has committed two evils: they have forsaken me the fountain of living water, and have dug for themselves broken cisterns. Jeremiah 2:13 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 w...
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; "
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.”