Romans 1:21
Romans 1:2121For 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.
Read as describing how humanity turned from the knowledge of Jehovah available in creation into idolatry and false religion — applied by the Watchtower to Christendom's departure from true worship. The Greek behind "God" here is not a subject of discussion in JW usage.
This verse has a quiet but useful role in the conversation. The NWT's defence of "a god" at c is that the Greek there has no definite article before theos, so it may be rendered indefinitely. has the same anarthrous theos, referring plainly to the Father, and the NWT renders it "God" — not "a god." So does every occurrence like it. You can make the point without any Greek by asking a Witness to consider what the rule would produce if applied consistently. In alone, theos appears without the article several times, and the NWT translates it "a god" exactly once — at 1:1c. That is a doctrinal selection, not a grammatical rule. The chapter also supplies a related word. Verse 20 speaks of God's eternal power and — in the NWT — his "Godship," rendering theiotes, the general divine attributes visible in creation. uses a stronger word, theotes, the essence of being God, and says all its fullness dwells in Christ bodily. Paul chose the weaker word for what creation shows and the stronger one for Jesus. Compare , .