God punishes them is supplied from God’s wrath in verse 18, to make clear the action implied by because. Men are punished because they sin even while knowing the truth about God. Is plain to them translates an aorist tense in Greek; Paul probably has in mind God’s actions in creating the universe, which he looks upon as being one event. Is plain to them may be rendered as “can be clearly seen,” “can be fully understood,” or even “there is no uncertainty about what men can know about God.”

What men can know about God is literally “what can be known about God,” and the context of this knowledge is indicated in the following verse: his eternal power and his divine nature.

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on Paul’s Letter to the Romans. (UBS Handbook Series). New York: UBS, 1973. For this and other handbooks for translators see here .