“Yes, they knew God, but they wouldn’t worship Him as God or even give Him thanks. And they began to think up foolish ideas of what God was like. As a result, their minds became dark and confused” (Romans 1:21 NLT).
Romans 1:21 offers an incisive look at those “…who knew all the time that there is a God, yet they refused to acknowledge him as such, or to thank him for what he is or does” (Phillips). According to this passage, the problem isn’t a lack of awareness regarding God’s existence. The issue is that we refuse to acknowledge Him despite our awareness of His existence. That refusal leads to an attempt to redefine “god” in ways that align with our “dark and confused” speculations concerning Him.
One source explains the crux of the issue…
“…it should be observed that Paul does not say that natural persons cannot perceive truth about God, but that they do not receive (Gk. δεκομαι, ‘welcome’) it. Paul emphatically declares that the basic truths about God are ‘clearly seen’ (Rom. 1:20). The problem is not that unbelievers are not aware of God’s existence. They do not want to accept him because of the moral consequences this would have on their sinful lives.” (1)
That decision yields another unfortunate response: ingratitude. When a gracious, benevolent Creator “…makes His sun rise on the evil and on the good, and sends rain on the just and on the unjust” (Matthew 5:45), we would do well to respond with an attitude of thankfulness and appreciation. Instead, we’re told that such individuals “became futile in their thinking” (ESV).
“Futility” denotes something that is useless and devoid of purpose. This definition is important, for our text from Romans 1:21 reveals a concern that is far more significant than it seems. For instance, consider the thoughts and activities that fill our days. Think about the way we invest our time, talents, and capabilities in the various affairs of life. There are many ideologies, beliefs, and philosophies that allow us to indulge in the belief that we are pursuing “the important things in life.” But apart from a genuine relationship with God in Christ, those pursuits ultimately lead to futility.
No reasonable person should choose to live a life of futility. No rational individual should approach the twilight years of life with the realization that he or she has failed to produce anything of lasting significance. Yet Romans 1:21 tells us that a refusal to act on what we know about God ultimately leads to that end.
(1) “Objections to Apologetics” Geisler, N. L. 1999. Baker Encyclopedia of Christian Apologetics. Baker Reference library. Baker Books: Grand Rapids, Mich

We can illustrate this function of the celestial heavens with the image of a train that is traveling down a flat set of railroad tracks. The last car of our hypothetical train is moving because it is coupled to the car ahead of it. The same is true of every other railroad car in that sequence; the motion of each individual car is explained by the action of the car that precedes it.
For instance, the visible world is like a mirror that reflects the existence and power of its Creator. The natural world offers a means of verifying God’s reality and a way to understand some of His attributes. Anyone with the gift of sight may enter the theater of nature and observe the glory of God through creation. That theater encompasses the world we inhabit along with the observable universe.
This reality has led to many creative forms of suppression among those who are uncomfortable with the reality of God’s existence. For instance, the term “Mother Nature” is a common way of referring to creation in a manner that eliminates the idea of a Creator. We might also refer to “luck,” “fate,” “chance,” or “karma” in an effort to camouflage or dismiss any consideration of God and His sovereignty over the affairs of life.
The word “suppress” refers to the act of holding something down. A common analogy for understanding this word is that of a sailor who is determined to pilot his or her boat against a strong current of water. To accomplish this, the sailor must work to “suppress” or counteract the natural flow of water by forcing the rudder to take the boat in the direction he or she wishes to go instead of the direction of the current.
That brings us to this reference to “godlessness,” a term that means “without reverence for God.” A godless person is someone who lives as if God did not exist, or exhibits irreverence for Him in thought, word, or deed. Much like a warning lamp that signals the presence of a fault condition, godlessness signals a defect in one’s vertical relationship with his or her Creator.