Because that when they knew God
Though they had such a knowledge of the being and perfections of God, yet
they glorified him not as God.
They neither thought nor spoke honourably of him; nor did they ascribe those perfections to him, which belonged to him; they did not adhere to him as the one and only God, nor honour him as the Creator of all things out of nothing, and as the sole Governor of the universe; they did not glorify him by the internal exercise of fear of him, love to him, or trust in him, nor by any external worship suitable to his nature, and their own notions of him, Seneca is an instance of this, of whom Austin F6 says,
``that he worshipped what he found fault with, did what he reproved, and adored that which he blamed.''Neither were thankful;
neither for the knowledge of things they had, which they ascribed to themselves; nor for their mercies, which they imputed to second causes:
but became vain in their imaginations;
the vanity or their minds was the spring and source of their evil conduct; which may design the wickedness of their hearts, and the imaginations thereof, which were evil, and that continually; the pride of their natures the carnality and weakness of their reasonings, and the whole system of their vain philosophy; and hence they ran into polytheism, or the worshipping of many gods:
and their foolish heart was darkened;
where they thought their great wisdom lay: darkness is natural to the hearts and understandings of all men, which is increased by personal iniquity; Satan is concerned in improving it, and God sometimes gives up the hearts of persons to judicial blindness, which was the case of these men.
F6 De Civitate Dei, l. 6. c. 10.