21 because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened.
This verse explains why humanity is without excuse (v. 20). Knowing God by revelation should result in His honor.
Rejection of clear evidence of God in creation shows human perversity, a twisting of truth so that people turned to idolatry. Their knowledge of God was not followed with willing worship.
although they knew God [had awareness of God by observing creation through personal experience; this is not a saving knowledge of God],
At one point people knew the God of creation but not the Savior. This culpability in knowing God made their rejection of that knowledge reprehensible.
they did not glorify Him as God,
A knowledge of God is not the same as glorifying Him as God. God made man to glorify Him. God’s glory is the manifestation of His greatness. The purpose of Creation is to glorify God and enjoy Him forever. These people knew enough about God to see that they should have glorified Him as Creator, yet they withdrew worship from Him. Unbelief produced this action. They should have worshiped Him for providing food, water, beauty in creation, and many other things.
nor were thankful,
Since people had come to a knowledge of God, they should have been thankful for His work of Creation. They did not act with a capacity of appreciation for Him; they were ungrateful for His creation.
but became futile [vain] in their thoughts [literally, deliberation with self, dialogue with self],
Their rejection of the God of Creation led them to “futile” thinking. The word “futile” means worthless, purposeless, pointlessness. They turned from thinking by divine revelation to their own empty theories about truth. Their thought life took a jolt.
The Greek word “futile” carries the ideas of vain, vacuum, void. The idea is that empty and evil notions were sucked into their minds to replace the notion of a Creator to whom they were responsible. Their human thinking was groundless and resulted in extreme skepticism. This was resignation to tragic existence. All that was left was a lament on the emptiness of existence. This was a plunge into nihilism (nothingness). It was a resignation of ever coming to truth and resulted in pagan thought life. Man became aimless with this thinking.
Eph 4:17, This I say, therefore, and testify in the Lord, that you should no longer walk as the rest of the Gentiles walk, in the futility of their mind,
Man with negative volition removed himself from the possibility of knowing that which gives everything its meaning.
By rejecting the knowledge of God in creation, people darken their hearts further. The problem with man is not that he is uneducated but that he hates God, hates His incursion into their lives. This hatred leads to a darkened soul towards God, the God who is the God of light. This has a consequence for epistemology (how we know what we know); we preempt the possibility of coming to a knowledge of God with certainty. Creation assumes a Creator and design demands a Designer.
In biblical assumption man does not move upward. He has proclivity to devolve, not evolve. If he goes negative towards God, the pull of his heart is downward. Even general revelation can befuddle man if he goes negative in volition (as those in Romans 1). There is a clear relationship of sin to unbelief. Sin has a blinding influence on the will. Evil seldom presents itself to us in its own ugly nature.