But what is Paul saying here? Throughout the opening chapters of Romans, Paul is laying the groundwork that all have sinned and fall short of God - both the Jew and the Gentile. To me, 1:18-20 isn't about people who haven't heard of Jesus, but about people who aren't aware of "the Law". The fact that they don't know the law is no excuse - they are still sinners and in need God's Grace. Go back and read the first few chapets of Romans (never a bad thing) and see if you don't agree...
From The Adam Clarke Commentary
Verse 19. That which may be known of God
Dr. Taylor paraphrases this and the following verse thus: "Although the Gentiles had no written revelation, yet what may be known of God is every where manifest among them, God having made a clear discovery of himself to them. For his being and perfections, invisible to our bodily eyes, have been, ever since the creation of the world, evidently to be seen, if attentively considered, in the visible beauty, order, and operations observable in the constitution and parts of the universe; especially his eternal power and universal dominion and providence: so that they cannot plead ignorance in excuse of their idolatry and wickedness."
Verse 20. The invisible things of him
His invisible perfections are manifested by his visible works, and may be apprehended by what he has made; their immensity showing his omnipotence, their vast variety and contrivance, his omniscience; and their adaptation to the most beneficent purposes, his infinite goodness and philanthropy.
From John Wesley's Explanatory Notes
on the Whole Bible
Verse 19. For what is to be known of God - Those great principles which are indispensably necessary to be known. Is manifest in them; for God hath showed it to them - By the light which enlightens every man that cometh into the world.
Verse 20. For those things of him which are invisible, are seen - By the eye of the mind. Being understood - They are seen by them, and them only, who use their understanding