21 because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened.
and their foolish [unintelligent] hearts [not emotions, but whole inner being—mind, emotion, and will] were darkened [passive—they received a firm state of rejection of God].
The word “foolish” carries the idea of morally meaningless. Rejecting truth darkens hearts further. Darkness was a characteristic of these people’s thinking. They carried a general rejection of God in their souls; they were no longer open to God in their volition.
Confronted with evidence of God, they refused to accept it. They apostatized from truth. All unbelief of revelation is apostasy. They formed an antithetical frame of reference and lost the light that they had.
Eph 4:18, having their understanding darkened, being alienated from the life of God, because of the ignorance that is in them, because of the blindness of their heart;
Light reveals blindness (Jn 1:5). It takes more than light to see, for it is necessary to have sight as well.
The purpose of our heart controls what we believe.
The set purpose of the heart controls what we desire to believe. It does not allow objective consideration to get in its way. Every new sin adds to the inveteracy of the heart. The heart keeps darkening further.
The predicament of man is not the absence of light but the rejection of light. It is a matter of revolt against and estrangement from God. The person “suppresses” truth for an altered motive (1:18). For this reason, man cannot discern truth in general revelation.
Jn 3:19-20, 19 And this is the condemnation, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil. 20 For everyone practicing evil hates the light and does not come to the light, lest his deeds should be exposed.
There is also a danger for Christians to reject God’s Word for their lives because they can enter into a state of rejection of God’s principles for their lives.