Romans 1:21b

For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened.

(v21, NIV)

What else does Paul tell us about these people? Not only are they futile in their thinking, but their hearts are darkened. What does this mean? Well, they exchanged the glory of God for images. From 1 John we know that God is light and in him is no darkness at all. If you trade him away, you no longer have him who is light. If you no longer have the light, where can you get light from? Within us is no light producing thing of any kind. There is only God, and without him, we walk in the darkness. Do not trade him away; otherwise your hearts will be darkened.

Therefore, since through God's mercy we have this ministry, we do not lose heart. Rather, we have renounced secret and shameful ways; we do not use deception, nor do we distort the word of God. On the contrary, by setting forth the truth plainly we commend ourselves to every man's conscience in the sight of God. And even if our gospel is veiled, it is veiled to those who are perishing. The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel of the glory of Christ, who is the image of God. For we do not preach ourselves, but Jesus Christ as Lord, and ourselves as your servants for Jesus' sake. For God, who said, "Let light shine out of darkness,"[a]made his light shine in our hearts to give us the light of the knowledge of the glory of God in the face of Christ.

(2 Corinthians 4:1-6, NIV)

Here in 2 Corinthians people are described as being blinded. The gospel is veiled to those who are perishing. The light refers to the light of the gospel, but they are filled with darkness and cannot see it. However, it is God who brings forth the light. It is only through God that light can shine within us who are filled with darkness. Do not trade away the glory of God which is the only light that can shine in your heart. Those who fall into idolatry and trade away his glory will have their hearts darkened.