Jamieson, Fausset and Brown on Romans 1:20

20. For the invisible things of him from--or "since"

the creation of the world are clearly seen--the mind brightly beholding what the eye cannot discern.

being understood by the things that are made--Thus, the outward creation is not the parent but the interpreter of our faith in God. That faith has its primary sources within our own breast (Ro 1:19); but it becomes an intelligible and articulate conviction only through what we observe around us ("by the things which are made," Ro 1:20). And thus are the inner and the outer revelation of God the complement of each other, making up between them one universal and immovable conviction that God is. (With this striking apostolic statement agree the latest conclusions of the most profound speculative students of Theism).

even his eternal power and Godhead--both that there is an Eternal Power, and that this is not a mere blind force, or pantheistic "spirit of nature," but the power of a living Godhead.

so that they are without excuse--all their degeneracy being a voluntary departure from truth thus brightly revealed to the unsophisticated spirit.