They knew enough of God to know that thanks and praise were due to Him; but neither of these did they offer. They put aside the natural instinct of adoration, and fell to speculations, which only led them farther and farther from the truth. The new knowledge of which they went in quest proved to be fiction; the old knowledge that 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 — reached 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 such as to enable them to dispense with the enlightenment from above.