Mental idolatry is when someone builds an idea about what God is like that is contrary to Scripture. That is just as much idolatry as bowing to an image. Just as shaping a metal image to represent God will misrepresent Him and make Him appear less than He truly is, a false mental image will create a mental idol and have the same affect. Failure to think rightly about God will lead us into the sin of idolatry. Man is created in the image of God. God is not created by the imagination of man.
A.W. Tozer said, “Among the sins to which the human heart is prone, hardly any other is more hateful to God than idolatry, for idolatry is at bottom a libel on His character. The idolatrous heart assumes that God is other than He is… Let us beware lest we in our pride accept the erroneous notion that idolatry consists only in kneeling before visible objects of adoration, and that civilized peoples are therefore free from it. The essence of idolatry is the entertainment of thoughts about God that are unworthy of Him…I believe there is scarcely an error in doctrine or a failure in applying Christian ethics that cannot be traced finally to imperfect and ignoble thoughts about God.”
Tozer is exactly right. When man rejects God as He is and instead creates God as he would like Him to be, it has terrible results.
“For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, 19 because what may be known of God is manifest in them, for God has shown it to them. 20 For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, 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.” Romans 1:18-21 (NKJV)
The picture here is that men recognize God’s revelation and then reject it. Actually, it says they suppress the truth. The idea here is that they repress the truth or that they hinder the truth from going forward. This is further illustrated when we are told that men knew God, or at least knew about God, and rather than submit to God they rejected Him. Their rejection of God is described in two ways.
First, they refuse to glorify God. They refused to worship God as Creator. They refused to come to God as Redeemer. They refused to submit to God as Lord. They refused to serve God as Worthy. The reason that man’s rejection of God leads them to refuse to glorify God is that they don’t like the things that are revealed about God. What is revealed about God teaches us something about His character and nature. What is revealed about the character and nature of God is something that man does not like.
Not only do men refuse to glorify God when they reject Him, but they also refuse to be thankful to God. Those who reject God and refuse to be thankful would probably say there is nothing to be thankful for because God, if there is a God, has never done anything for them. Of course, they are ignoring, either willfully or ignorantly, the common graces God pours out on all people.
Jesus says that God causes rain to fall on the just and the unjust (Matthew 5:45). God does any number of things every day for all people. He makes the sun to rise. He causes rain to fall. He gives people the intelligence to develop technology and medicine. He gives us the breath of life. Every good and perfect thing in this world comes from God (James 1:17). But men who reject God refuse to acknowledge God’s role in these things and so refuse to give Him thanks.
The result of rejecting God is that men become futile in their thoughts about God. Futile sorta means empty and carries with it the idea of wrong or worthless. Instead of receiving the revelation about what God is truly like, they begin to make up empty, wrong and worthless ideas about what God is really like. Rather than worshipping the great and glorious God as He is, they come up with foolish ideas about what God is like. They get rid of all the things about God that make them uncomfortable and remake God in an image they like better. A kinder, gentler god that makes them feel all warm on the inside. Unfortunately for them, what they’ve done is rejected God and become futile in their thoughts.
They also become darkened in their hearts. The word for darkened used here pictures blindness. They have become blinded to the truth about God. They are in spiritual darkness and cannot see the glory of God in the face of Christ Jesus (2 Corinthians 4). They find the Gospel to be foolish or offensive. They cannot believe that anyone would find the Bible relevant for their lives today. They are lost in spiritual darkness without any light to guide them to find a way out of the darkness. They are helpless and hopeless on their own.
That is a pretty bad downward trajectory. The rest of chapter 1 gives more detail about how far man pushes into depravity when they reject the revelation about God. This is why it is important that we understand and accept God as He is. If we want to know God we must accept God as He is and not try to remake Him into an image we are comfortable with. When we do this, we are not changing God, we are simply creating an idol that will keep us from knowing God.
For further study read Exodus 32. When Aaron presented the calf to Israel who did he say the calf was? What was the result of this mental idolatry on the people? What was God’s attitude toward this mental idolatry? How do we see mental idolatry today?