What may be known about God is plain to them (Romans 1:19–21)

Says "What may be known about God is plain to them, because God has made it plain to them" (1:19, BSB). Implies God is the one who reveals here too. His disclosure in creation is his own act, and it is not a human achievement of reason. The word order ὁ θεὸς γάρ makes no difference to the sense (var.Rom.1.19.ho-theos-gar).

Says "For since the creation of the world God's invisible qualities, His eternal power and divine nature, have been clearly seen, being understood from His workmanship" (1:20, BSB). Implies The paradox is deliberate: "invisible things … are seen." The phrase νοούμενα καθορᾶται (nooumena kathoratai) means "being perceived with the mind, they are clearly seen." The seeing is a seeing by the mind, through the things that have been made (note.language.Rom.1.20.nooumena-kathoratai; lex.G2529). What is known is specific and limited: his "eternal power" (ἀΐδιος δύναμις) and his "divine nature" (θειότης, theiotēs, the quality of being divine, not θεότης, "deity"). It does not include his saving purpose, which is made known only in the gospel (1:16–17; 10:14–17). Implies "Since the creation of the world" and "workmanship" (ποιήμασιν) point to Gen 1:1, "In the beginning God created the heavens and the earth." The ordered creation of Gen 1 is the "work" that witnesses to its Maker (compare Ps 19:1–4; Acts 14:17; note.language.Rom.1.20.gen-1-creation-order; xref.Rom.1.20.Gen.1.1.heb).

Says The purpose or result is εἰς τὸ εἶναι αὐτοὺς ἀναπολογήτους, "so that men are without excuse" (1:20, BSB; note.language.Rom.1.20.anapologetous; lex.G379). Implies The revelation in creation is enough to make people accountable but not enough to save them. Romans 2:1 uses the same word for the person who judges others (ἀναπολόγητος), and that passage extends the charge to all.

Says "For although they knew God, they neither glorified Him as God nor gave thanks to Him, but they became futile in their thinking and darkened in their foolish hearts" (1:21, BSB). Implies "Knew" (γνόντες) is real knowledge. It is not saving knowledge, but it is knowledge that is refused. The root sin is a failure of worship and thanks, and the darkening of mind follows it as its consequence. The judgment does not come first and cause the refusal. The verb ἐματαιώθησαν ("became futile") echoes the Old Testament language for idols as "vanity" (Jer 2:5; 2 Kgs 17:15 LXX, ἐπορεύθησαν ὀπίσω τῶν ματαίων καὶ ἐματαιώθησαν).

A parallel argument in the Wisdom of Solomon (non-canonical; cited for historical context only) 13:1–9 is close to Paul: from the good things that are seen, people were not able to know "the One who is" (τὸν ὄντα, 13:1, literal). Paul's charge goes further. Wisdom grants that for such people "the blame is small" (μέμψις ὀλίγη, 13:6, literal), while Paul says that they knew and refused (note.historical.Rom.1.1-Rom.1.32.wisdom-polemic).