The main problem with the behaviour of the Gentiles was not their outward practices, although these practices were evil. These practices were the visible symptoms of an inner disease. Paul describes a movement from how they thought, to how they felt, to how they behaved.
He describes their minds as empty, darkened and ignorant. The reason why their minds were like this was the blindness of their hearts. The word that is translated ‘blindness’ has the idea of hardness and callousness. The term ‘heart’ does not refer to one’s emotions, which is how we use the word today. Rather it refers to the entire inner man, which Paul divides here into mind, feelings and choices. It was the depth of rebellion against God within each person that was the major problem.
Obviously Paul does not mean that their minds were a total blank, incapable of thinking. Instead he means that the Gentiles did not think of God. It is common in the Bible for idols to be described as vanity and this is probably what Paul has in mind when he says the thoughts of the Gentiles were futile. They worshipped false gods who did not exist.
When a person thinks of God, it gives solidity, insight and information to their minds. As Leon Morris put it, ‘There is far more to being a Christian than intellectual achievement, but we should be clear that being a follower of Christ means using the brains God has given us to their fullest capacity.’
But it is not only their minds that were affected by sin. Paul comments that the Gentiles had lost insensitivity with regard to their behaviour; they were shameless, indicating that the Gentiles were not embarrassed by their sins.
This is what life becomes without knowledge of God, of his character and of his purposes. And Paul’s description is not limited to his own day, for it also describes ours. We live in a society that loves to sin openly and continually.
The main reason for people’s behaviour is the gods they worship. In our society, self-indulgence (when pleasure becomes god) and materialism (when things become god) dominate many lives. There are many more such gods and no doubt you can list them for yourselves. But our minds and hearts were made for higher experiences, of knowing God experientially through Jesus.
The remedy is to learn about Jesus and from Jesus.