Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
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.
Vs.21
Because that when they knew God - When
they thus acquired a general knowledge of the unity and perfections of
the Divine nature, they glorified Him not as God - they did not proclaim
him to the people, but shut up His glory in their mysteries, and gave the people, in exchange for an
incorruptible God, an image made like to corruptible man.
---Wherefore God,
in punishment for their sins, thus turning his truth into a lie,
suffered even their mysteries, which they had erected for a school of
virtue, to degenerate into an odious sink of vice and immorality; giving
them up unto all uncleanness and vile affections.
Neither were thankful - They manifested no gratitude for the 
blessings they received from His providence, but became vain in their imaginations, διαλογισμοις,
in their reasonings.
---This certainly refers to the foolish manner in
which even the wisest of their philosophers discoursed about the Divine
nature,.....Who can read their
works without being struck with the vanity of their reasonings, as well
as with the stupidity of their nonsense, when speaking about God?
But they became vain in their
imaginations - en tois dialogismois—in
their reasonings, in their practical inferences.
---The several opinions and hypotheses of the various sects of
philosophers concerning these things were so many vain imaginations.
In short, their foolish, darkened minds sought God no where but in the
place in which He is never to be found; the vile, corrupted, and
corrupting passions of their own hearts.
