Because humans suppress the truth and do not honor God, they became unable to respond properly to God (v. 21a). The verb “render futile” (ματαιόω) is used of idolatry (Jer 2:5) and has the sense of emptiness or worthlessness. The word-group is used to describe idols as worthless things. Several commentaries suggest the possibility of an allusion to Psalm 94:11 (LXX 93:11), “the thoughts of man are worthless.” Kruse, Romans, 96, for example. Although the form of the word is different (LXX Ps 93:11 has a noun rather than a verb), that both texts combine a word from the ματαιόω word group and διαλογισμός makes this allusion probable.
What has been “rendered worthless” is humanity’s thinking. The noun here (διαλογισμός) refers to discussions or arguments, the “content of reasoning or conclusion reached through use of reason” (BDAG). The idol-worshiper has a logical, rational reason for worshiping something which is not worthy of worship, but that reasoning is itself futile.
The hearts of those who suppress the truth are foolish and darkened (v. 21b). The heart is the place where one thinks and reasons (not the head). The word Paul uses is not the common word for foolishness but the rare word ἀσύνετος (asynetos). It is used only here and 1:31 (Matt 15:16/Mark 7:18, not understanding Jesus’s teaching).
This noun has the sense of “lacking understanding” (BDAG), but also a lack of moral character (TestLevi 7:2). An inscription at Ephesus uses this word with the sense of “stupid,” but Moulton and Milligan comment that “it seems clear that “foolish” here does not primarily denote lack of brains but moral obliquity” (MM 87).
To become darkened (σκοτίζω) is also used for “moral darkening” in Second Temple period literature.
TestReub 3.8 And thus every young man is destroyed, darkening his mind from the truth, neither gaining understanding in the Law of God nor heeding the advice of his fathers…
TestLevi 14.4 For what will all the nations do if you become darkened with impiety?
This moral darkening is the reason the Gentile world practices idolatry. Humans became fools by exchanging the knowledge of the creator for images of creation (v. 22-23). They claimed to be wise, but they became fools when they worshiped creation rather than creator. To worship a god that looks like a human is foolish, but at least a human is in the image of God. To worship other created things (birds, animals and reptiles) is even more foolish since they were not made in the image of God in the first place (Kruse, Romans, 97).
In describing idolatry as foolishness, Paul does not depart at all from the prophetic condemnation of idolatry (for example, Isa 44:13). Paul may be alluding to Psalm 106:20, “They exchanged the glory of God for the image of an ox that eats grass” or Deuteronomy 4:15-18. In the context of that Psalm, the wilderness generation “forgot their God and Savior” and what he did for them at the Red Sea. Because they exchanged that knowledge for foolishness, they fell under God’s wrath (ὀργή, cf. Rom 1:18).
It is remarkable Paul would describe worship practiced by the entire world at that point in history as “foolishness,” but even some Greek and Roman writers who considered the worship of gods to be foolish. Although describing someone’s religious beliefs as foolish is not polite in the modern world, Paul is not far from his contemporaries in mocking the worthlessness of worshiping idols.
Once again, I wonder how well this “works” in modern presentations of the Gospel. In the modern west, dismissal of gods and idols is passed over quickly since few would consider worshipping an idol. But for the majority world, this is a serious question. How can the Gospel be presented to a world which does worship a variety of gods and idols in a way which dismisses the gods yet still attracts people to the Gospel? For example, how do Asian Christians deal with veneration of ancestors? I would love to hear from readers in non-Western countries on this issue: How is Romans 1:21-23 taught and preached in cultures which are dominated by worship of gods?