That the ungodly know God means
from creation, the ungodly know they ought to worship and thank God; or
from creation, the ungodly do not gain enough knowledge to be in a relationship with God.
What Paul means is that God reveals himself through creation so that humans ought to know enough about him to worship and thank him. We can detect that this is Paul’s meaning for two reasons. First, there was a common Jewish understanding that to know God is to be obliged to acknowledge him. We notice this line of thinking in Judges 2:10–11, which says that the generation after Joshua does not know God, in the sense that they do not acknowledge God. Relatedly, the psalmist writes in Psalm 79:6 that the nations do not know or call upon God, implying that to know God is to acknowledge him.
Second, Paul says at the end of Romans 1:20 that the ungodly are without excuse, and then explains the reason why in Romans 1:21. They are without excuse because they know God but do not worship or thank him.
Others contend that Paul is saying that natural revelation is not enough for humans to acknowledge God. In other words, while God reveals himself through natural revelation, the ungodly still do not worship or thank him. That’s because natural revelation is not enough to know that one ought to praise and worship God. These point out that according to Paul, the reasoning of the ungodly becomes futile and their hearts are hardened. In other words, while God reveals himself through creation, their incapacity to reason and their hardness of heart prevent them from praising and thanking God.
The problem with this argument is that Paul does not say that because of their futile thinking and hardness of heart they do not acknowledge God. Rather, he says that because they know God and yet do not worship and thank him, their reasoning becomes futile and their hearts harden. In other words, it is the result of not following through on their responsibility to acknowledge God, which they know from natural revelation they ought to do, that their thinking becomes futile and their hearts hard.
Thus, when Paul says they knew God but did not acknowledge or thank him, he is saying that the ungodly can know enough about God through creation to worship and thank him.
Interpretation 1:
From creation, the ungodly know they ought to worship and thank God.
Summary:
Paul explains that God revealed enough about himself through creation that humans know not only that he exists but that we ought to worship and glorify him. That humans do not respond to this knowledge of God demonstrates our selfishness and makes us without excuse.
God creates the universe and by doing so reveals his glory and power. When we consider the excellence and majesty that God must have in order to design such a glorious creation, we ought to worship and thank him.
Advocates:
James Dunn
Craig Keener
Richard Longenecker
John MacArthur
Leon Morris
Frank Thielman
Minor differences:
Our authors agree that through natural revelation, humans not only know God but know that they ought to thank and worship him.
For James Dunn, the fact that Paul says they know God and ought to honour and thank him suggests that Paul is moving into a Jewish train of thought. The notion is that in Jewish thinking, to know God is not just to perceive God as he is, but to be motivated to worship him.1 Frank Thielman agrees with Dunn, explaining that from a Jewish perspective, creation should prompt all people to glorify God.2
Richard Longenecker does not mention whether or not Paul’s line of argument is influenced by Judaism. Rather, he focuses on the fact that according to Romans 1:21, Paul is saying that natural revelation reveals not only that God exists but that he ought to be worshipped.3 So it is not the knowledge that God exists that means people are without excuse, but the knowledge that God exists and ought to be glorified.
Arguments
Possible weaknesses
Interpretation 2:
From creation, the ungodly do not gain enough knowledge to be in a relationship with God.
Summary:
Paul explains that although humans knew God from creation, they did not worship or give thanks to him. This shows that natural revelation through creation is not enough to bring one into a relationship with God.
Advocate:
Douglas Moo
Arguments
Possible weaknesses
21 For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened.