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.
Because that, when they knew God (in the sense of Romans 1:19) - even while still retaining some real knowledge of God, and before they sank down into the state next to be described,
They glorified him not as God, neither were thankful - neither yielding Him the adoration due to Himself, nor rendering the gratitude which His beneficence demanded,
But became vain in their imaginations, х en (G1722) tois (G3588) dialogismois (G1261)] - 'in their reasonings,' 'thoughts,' 'speculations' about God, The word rendered, "became vain" х emataiootheesan (G3154)], and the corresponding word, 'vanity' х mataiotees (G3153)], almost always refer to the idolatrous tendencies and practices of men (Jeremiah 2:5; 2 Kings 17:15; Acts 14:15). The word rendered "imaginations" is mostly used in a bad sense, and here refers to men's proud and restless dissatisfaction with the simple verities regarding God, which are "manifest in them," their cravings after something more satisfactory, and the thoughts, reasonings, or speculations to which these gave rise.
And their foolish heart, х asunetos (G801)] - 'their senseless,' 'stupid heart;' meaning their whole inner man,
Was darkened. How instructively is the downward progress of the human soul here traced! When once darkness is suffered to overspread the mind, an impotent stupidity of all the active powers of the soul is the result; and thus the truth which God left with and in men, instead of having free scope to acquire strength and develop itself, came by degrees to be lost, and the still, small voice of conscience, first disregarded, was next thwarted, and at length systematically disregarded.