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.
Ver. 21. Neither were thankful] How then shall we answer to God our hateful unthankfulness, which is (saith one) "a monster in nature, a solecism in manners, a paradox in divinity, a parching wind to dam up the fountain of divine favours." Woe be to our Solifugae that abuse gospel light; these put not light "under a bushel" (as the poor Paynims did) but under a dunghill; Gravis est lux conscientiae, Heavy is the light of conscience, saith Seneca; but heavier is the light of the gospel, gravior est lux Evangelii, say we. A heavy account will they give that abuse the light of nature; but much heavier they that "receive the grace of God in vain."
But became vain in their imaginations] Gr. διαλογισμοις, in their reasonings, disputations, discourses upon serious deliberation. They stood not to their own principles (as, that there is one God only, that this God is to be worshipped, &c.), but were atheists by night that worshipped the sun, and atheists by day that worshipped the moon, as Cyril saith wittily.