v. 18b τῶν τὴν ἀλήθειαν ἐν ἀδικίᾳ κατεχόντων (“suppress the truth in unrighteousness”): Adam received the law of God yet he did nothing to defend his wife Eve while she was being deceived by the serpent.
v. 19 διότι τὸ γνωστὸν τοῦ θεοῦ φανερόν ἐστιν ἐν αὐτοῖς, ὁ θεὸς γὰρ αὐτοῖς ἐφανέρωσεν (“because that which is known about God is evident within them; for God made it evident to them”): Who personifies this reality better than Adam. While the Apostle is writing his condemnation of humanity in general in vv. 18-32 it should be noted that Adam is the paradigm. He was the only person who can really, really fit this description while rejecting God. Everyone else has very indirect interaction with God in contrast.
v. 2ob ἀπὸ κτίσεως κόσμου (“since the creation of the world”): While it would appear that this indictment covers everyone (and I think it does) there is only one person who was around at the very beginning of the biblical narrative: Adam. No one else would have known the “power” and “deity” of God like him.
v. 21 διότι γνόντες τὸν θεὸν οὐχ ὡς θεὸν ἐδόξασαν ἢ ηὐχαρίστησαν ἀλλὰ ἐματαιώθησαν ἐν τοῖς διαλογισμοῖς αὐτῶν καὶ ἐσκοτίσθη ἡ ἀσύνετος αὐτῶν καρδία (“For even though they knew God, they did not glorify him as God or give thanks, but their thinking became futile and their foolish hearts were darkened”): Adam (and Eve) knew God as God. Yet they listened to a serpent instead. This is a denial of God as God. Their obedience toward the serpent was motivated by their desire for the one thing that God denied them: the Tree of the Knowledge of Good and Evil. They ignored everything else that God had given them. There is no better way to describe this than that they were simply not thankful. This resulted in “futile thinking” and a “darkened heart”.
v. 22 φάσκοντες εἶναι σοφοὶ ἐμωράνθησαν (“Although they claimed to be wise, they became fools”): The serpent tricked them into seeing themselves as being able to know things like God knows things. They assumed that they were able to handle the knowledge of good and evil. They affirmed the serpent when he proposed that maybe God didn’t want them to be like him. In the end, they became fools.
v. 23 καὶ ἤλλαξαν τὴν δόξαν τοῦ ἀφθάρτου θεοῦ ἐν ὁμοιώματι εἰκόνος φθαρτοῦ ἀνθρώπου καὶ πετεινῶν καὶ τετραπόδων καὶ ἑρπετῶν (“and they exchanged the glory of the immortal God for images made to look like mortal human being and birds and beast and reptiles”): It is here we began to see the transition toward “Adam as humanity”. It appears to me that the “glory of the immortal God” is another way of describing what we would call the imago Dei or what Gen. 1.26 describes as the “image…and likeness” of God. In 2.7 those who seek “immortality” receive it. This seems to indicate that humans get to share in the “immortality” of God that they once forsook.
Likewise, these categories match the LXX in Gen. 1.20-27. We find the birds, the beast, the reptiles (or “creeping things”), and, of course, humanity. What Adam and Eve were created to rule it appears they worshiped according to the Apostle. It would seem that the best example of this was their obedience of the serpent rather than they “subduing” of creation (cf. Gen. 1.28). For Paul this obedience is reenacted whenever humans create idols worshiping the animals they were supposed to control in honor of the Creator (see v. 25b as well)
v. 25a οἵτινες μετήλλαξαν τὴν ἀλήθειαν τοῦ θεοῦ ἐν τῷ ψεύδει (“They exchanged the truth of God for a lie”): While the grammatic construct ἐν τῷ ψεύδει can mean “for a lie” because it is part of a prepositional phrase, it would seem to me that even if we use the indefinite article there is a particular “lie” in place. Another option would be that there is a general “error” into which all humans fall. Yet the flow of this whole passage makes me think that whether we say “the lie” or “a lie” this whole thing is informed by Gen. 3.4 where the serpent tells Adam and Eve that God was wrong and that their disobedience will not lead to death.
In v. 32 Paul clearly points out abouts the sins he just listed “Although they know God’s righteous decree that those who do such things deserve death, they not only continue to do these very things but also approve of those who practice them.” Yes, the “righteous decree” is embodied in the Law (for the Jew, 2.12b) first and foremost, yet there is a “law” that has been embedded in the human psyche that holds all humans accountable (see 2.12a). Adam’s reception of God’s commandment in Eden is an good example of this.