Romans 1:21-23, "For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things."
First, in verse 21 we see that man doesn’t glorify God in his natural state. Though it is the primary purpose of man to glorify God and enjoy him forever, as a result of his sinfulness man would rather worship themselves or something altogether other than God.
Secondly we see in the latter part of this verse that the unconverted man is not thankful. Or, to say it another way, man does not offer honor or thanks which is appropriate for his everlasting power and Godhead .This implies the reality that man should be of a thankful heart due to his being created and sharing in the delights of God’s creation. Nevertheless, man has no reason to be thankful for something (or someone) he believes fails to give credence to.
Thirdly, man becomes vain in their imaginations, or as the ESV renders it, futile in their thinking. When man removes God from his thinking he becomes entirely other than he was created to be. Proverbs 1:7 tells us that the fear of the LORD is the beginning of knowledge. Therefore, if man is to acquire anything he must first acknowledge God in his thinking.
Fourthly, the text shows us that man allows their foolish hearts were darkened. As Matthew Henry notes, nothing tends more to the blinding and perverting of the understanding than the corruption and depravedness of the will and affections. Light and darkness is a concept regularly used in the NT, especially of Paul and of John. Luke tells us that Jesus came to give light to those who sit in darkness and in the shadow of death. Paul is clearly making this the object of his thinking in this verse.
Fifthly, man professes to be wise. Quite opposite of the truth, the unregenerate man confuses himself so much that he actually thinks he is wise. A parallel and subsequent concept to this is the connection of a person’s fallacious wisdom to that of the proverb knowledge puffs up. When man acquires what he supposes to be knowledge, he often fails to compare his undersized capacity to that of the omniscient God. Satan has done the same thing. Given the stature of the highest of angels, he abused this role and began the rebellion; to which fallen mankind falls right in line with.
Sixthly, and this is the truth about the man who falsely professes wisdom, that he has become a fool. The truth of the matter of man claiming to be wise is that he in fact has become a maroino, that is, he has become foolish. Paul is evidently alluding to the erroneous nature of mankind in involving themselves in idolatry. A fool is one who has turned away from God to his own way. In this case, he has done so as a result of his willing removal of God in his thinking.
Lastly, in verse 23 we see the ultimate result of the preceding notions, namely, that man makes for himself his own gods. The text says, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and reptiles. Man has so utterly removed God from his thinking, but retains the need for worship, that he cannot face the Omnipotent God because of his sin, so he invents his own god to supplant the living and true one. As John Owen has said so eloquently,
since men have become sinners, they require, but by nature lack of sin, of conviction, and of repentance toward God. If anyone will ground all of his hopes on a correct interpretation of God's self-revelation in providence, they simply cannot hope to come to an understanding of Him, of how He is to be worshipped, or of how their lives might be framed so as to be pleasing to Him. Why? Because sin stands in the breach between them! No revelation which lacks the elements of conviction, repentance, or atonement can suffice for fallen man! Will any man attempt to forge a system by which the pagans have so ordered their lives by the light that they had as to obtain salvation? He has his work cut out indeed! Any number of such reasoning might be brought forward with no effect at all. (Biblical Theology, SDG, 55)