Charles Ellicott Commentary Romans 1:21

"because that, knowing God, they glorified him not as God, neither gave thanks; but became vain in their reasonings, and their senseless heart was darkened." — Romans 1:21 (ASV)

They knew enough of God to know that thanks and praise were due to Him, but they offered neither of these. They put aside the natural instinct of adoration and fell to speculations, which only led them further and further from the truth.

The new knowledge they sought proved to be fiction; the old knowledge they had was obscured and lost by their folly. Starting with two things—a portion of enlightenment on the one hand, and the natural tendency of the human mind to error on the other—the latter prevailed, and the former became eclipsed.

But became vain in their imaginations.—They were frustrated, reaching no good and sound result with their speculations.

Their foolish heart.—Not the same word as “fools” in the next verse. Their unintelligent heart; their heart which, by itself, was endowed with no special faculty of discernment that would enable them to dispense with the enlightenment from above.