God can be understood—even the unconverted can comprehend some things about Him. Despite these verses in Romans 1, the opinions of learned men say that God is incomprehensible, yet Paul is saying that there is a clear testimony. It is a constant and natural revelation of God's power and nature, and that revelation is sufficient for God to hold these people responsible for their conduct.
This natural revelation, however, is not sufficient for salvation because God shows in other places that salvation requires a specific and personal revelation of His word. "No one," Jesus says in John 6:44, "can come to Me unless the Father who sent Me draws him; and I will raise him up at the last day."
But this revelation through what God has created is clear enough for Him to hold people responsible for their conduct. Thus, if His invisible attributes, eternal power, and divine nature are clearly understood by the visible things that God has made in this world, then all we need to do is to use a little common sense in connection with plain statements from Scripture to find out what God really looks like. So, if God says that His attributes can be clearly understood by the unconverted, and if He is seen in the visible creation in this world, what visible things on earth give us a picture of the invisible God?
The very thing that God Himself says in Genesis 1:26. We—mankind—look like Him.
Is that so difficult? Just understanding this principle, it is no wonder that the Greek gods of mythology reflected mankind in all of our foibles, weaknesses, and passions. The Greeks simply turned the principle around. They turned the image around, reflecting in their gods the things of man.
Other portions of Scripture, like I Corinthians 2:6-16, explain the special, personal revelation of God that helps us to know the things of God, so that we can have the mind of Christ and put on His image. However, we know from other passages that the created human being is but a pale reflection of the reality of God, and that God's creative power is still at work reproducing His image in men. That is, we are a work in progress and still unfinished.