An excerpt from "Bible Ref.com" entitled, "What does Romans1:20 mean?" states: "Paul asserts that human beings can easily know at least some things about God by looking at creation. We should look at what is visible around us in nature, what God has made, and arrive at some obvious conclusions about what is not visible. Adding one and one together, we should understand from nature that God has eternal power and a divine nature. David said something similar in Psalm 19:1–6.
After all, Paul seems to be saying, what kind of power would it take to make the world and all that is in it? Such a feat would require "eternal power," or endless and inexhaustible power. Such a Creator must also be divine and not merely human. He must be God, in other words. Human beings should look at creation and decide there must be a God who made it, a God we must answer to on some level." This article may be viewed at: What does Romans 1:20 mean?/bibleref.com
Reference: Romans 1:20