Romans 1:21

‘Because that, knowing God, they glorified him not as God, nor gave thanks, but became vain in their reasonings, and their senseless heart was darkened.'

Note that it was not that men did not ‘know' God. There was something within them which made them aware of Him. That is why there is so much religion in the world. But what they did not want was to be controlled by Him in their activities and behaviour. Thus they closed their minds to the knowledge of God as He is, and refused to glorify Him as God. Note that it is seen as deliberate. True knowledge of God was not seen as convenient. Nor did they render Him thanks. Note the emphasis on the fact that they were ungrateful. They took what He provided for granted, but would not acknowledge it. So instead they became vain and empty in their thinking and in their reasonings as they sought to find ways to satisfy the emptiness within, without recourse to God. But the result of rejecting the light was that their senseless heart was darkened. They found themselves struggling in the dark and sought to come up with a solution which would satisfy their desires and the desires of the masses, without having to face up to the truth.

The word for ‘vain' is used elsewhere to indicate a ‘corrupt' manner of living (1 Peter 1:18), while ‘vanity of mind' results in men being hardened and giving themselves up to various types of sin (Ephesians 4:17). So their vain reasonings were not just empty or futile reasonings, they were positively sinful. A related word is constantly used in the Old Testament in connection with idolatry. Such sin led to idolatry.