21 Because that, when they knew God, they glorified Him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
Man knew God was God but willfully determined not to glorify Him. Humanity began with a knowledge of God. They accepted God’s creation but weren’t thankful for it.
Vain = useless. Men became futile for the purpose for which they were created.
Imaginations = self-willed, evil reasonings in opposition to facts revealed by God about God. Since man rejected God, he had to invent man-made philosophies to replace Him. This plunged them deeper into darkness.
Darkened — spiritual darkness. Ephesians 4:17-18 — This I say therefore, and testify in the Lord, that ye henceforth walk not as other Gentiles walk, in the vanity of their mind, having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart.