First, the object of their knowledge is God’s “invisible attributes.”
Second, he explains the location of their knowledge of these invisible divine attributes: “in the things that have been made.”
Third, he explains the duration of their knowledge, to the effect that this has always been the case: “ever since the creation of the world.”
Fourth, he points to the quality of their knowledge: it is “clearly perceived,”
Paul adds all of this together and draws the inescapable conclusion (oun, so, therefore) for those who know God but suppress his truth: “they are without excuse.” None can plead ignorance, therefore none can excuse their moral responsibility and culpability.
Paul continues to explain what he means in verse 21. Their knowledge of God should lead to two appropriate responses, but instead we see two regrettable reversals: (1) they refused to honor God as God and (2) they refused to thank God for his wonderful gifts.
Read the whole article for the details.
I commented that this is one reason I don't believe in atheists. I don't believe they exist.
To explain this thought:
On the one hand, I love the irony of claiming to not believe in people who believe they don't believe in the existence of God. Simply claiming God or someone doesn't exist doesn't mean that they do. But the truth is that they do know of His existence and have chosen self-delusion in an attempt to escape His sovereign right over them.
Justin ended the article with this:
Studying just these few verses gives us enormous insight into what the pagans know and why they are responsible. May it motivate us to bring the gospel to those who are both near and far.
I couldn't agree more. Paul wrote this section for a reason. I can imagine that atheists would be incensed to be told that they really know God down deep and have simply suppressed that knowledge. However, if the Holy Spirit so enlightens a person of their internal motives for denying God, then this passage could be used by Him as a means for that enlightenment, and subsequent conviction.
So perhaps the information Paul gives in this passage is indeed evangelistic in nature. Do not fear causing someone to be incensed if it may change their heart.