We know sin is any violation of the law of God, and we’re never going to be sensitive to sin unless we are constantly made sensitive through the teaching of the Word. Because the culture is just drowning people, including Christians, in this new morality and new psychological explanation for iniquity, keeping a pure life is very, very challenging.

But let’s go a little deeper. What sin is most serious?

The medieval theologians had it right: It’s the sins of the mind. Jesus said it: It’s out of the heart that the mouth speaks. It’s what’s in the man’s heart that comes out that is so defiling. The seven deadly sins of medieval theology were not behaviors at all; they were sins in the mind. And no sin is more destructive to the conscience than the sin that takes place in the arena of the mind, because the conscience is the only deterrent.

A Christian friend can be a deterrent to a sin of the tongue. You’re going to watch what you say if you’re around a Christian. But the only deterrent that you have to the sins of the mind is your conscience. And you need to feed the Word of God constantly into your mind so that your conscience operates with full power. It needs to be so sensitive to the sins of your mind that you can enjoy what Paul enjoyed when he said, “Our proud confidence is this, the testimony of a good conscience.”

You’ve got to deal with the sins of the mind. 1 Corinthians 2:11 says, “For who among men knows the depths of a man except the spirit of the man which is in him?” Only you and God know about them.

Many people who won’t do evil deeds are nevertheless boldly evil in their thoughts. A man who, for example, abstains from fornication for fear of getting caught might convince himself that it’s okay to indulge in salacious fantasies because no one will ever discover such a private sin. The fact of the matter is that the sin he deliberately entertains in his mind may be a thousand times more evil than anything he would ever think of doing before others. And Scripture says his guilt is the same before God as if he acted it out.

You see, to indulge in sins of thought is to molest your conscience directly, to have unending guilt and the absence of joy. The guilt is inherent in the evil thought. “To the pure,” said Paul in Titus 1:15, “all things are pure, but to those who are defiled and unbelieving, nothing is pure, but both their mind and their conscience are defiled.” Nothing damages the conscience more than the habit of indulging in evil thoughts.

Unfortunately, once it’s begun, the practice becomes all too easy. This is sin that doesn’t need any opportunity.

Have you noticed that? It can happen anytime, anywhere, under any circumstance, and that is why when you begin to cultivate sins of the mind, you are putting yourself in an absolutely terrorizing situation because you can’t escape it. You think that it’s okay because it’s not on the outside. The truth of the matter is it is worse, because it is undetected by others and therefore breeding habitual iniquity by engaging the mind, the emotions, the desire, and the imagination.

Sinful habits follow a flow. Sow a thought, reap an act; sow an act, reap a habit; sow a habit, reap a character; sow a character, reap a destiny. It’s a tragic thing. And that is why it is so important that you hear the Word of God and are sensitized to sin constantly. People can go to many churches over and over and sin is never confronted. That may make them feel momentarily comfortable, but it does nothing for their long-term conscience. Eventually, those things on the inside will show up on the outside.

No one ever falls into adultery. The adulterer’s heart has been shaped by a long process of lustful thoughts. The heart of the thief is bent by covetousness long before his act of thievery. And James says in James 1:13-14, “Let no one say when he is tempted, ‘I am being tempted by God’; for God cannot be tempted by evil, and He Himself does not tempt anyone. But each one is tempted when he is carried away and enticed by his own lust.”

Don’t be deceived, brethren. It all starts inside.

Again and again Christ rebuked the Pharisees because they observed the external ceremonial law and neglected the moral part. They were utterly preoccupied with appearances and they were like a tomb: white on the outside, but on the inside they stunk with dead men’s bones. He says, “Woe to you, scribes and Pharisees, hypocrites! For you clean the outside of the cup and of the dish, but inside they are full of robbery and self-indulgence.”

What should be going on in the deepest recesses of our minds and hearts? I’ll tell you what: worship and love to God. When we were saved, we were saved to be true worshipers. To sin in the mind, then, is to desecrate the very sanctuary where our highest and best worship should be taking place. Cultivating sin not only defiles the mind, but it displaces worship for which we were saved.

It is relatively easy sometimes, to confess and forsake deeds or words of sin, but the sins of our thought life go unconfessed more than any other kind. They are the soul-blackening, the character-damaging sins. They work directly against the conscience. That’s why the Old Testament says in Proverbs 4:23, “Guard your heart with all diligence, for from it flow the springs of life.”

But 1 John 3:20 says, “God is greater than our heart and knows all things.” God knows whether we have a lusting, coveting, angry, selfish, proud heart that is cultivating those thought sins, or whether our heart is given over to worship Him. Jesus told the Pharisees in Luke 16:15, “You are those who justify yourselves in the sight of men, but God knows your hearts, for that which is highly esteemed among men is detestable in the sight of God.”

What’s going on in your heart is the litmus test of your character. Proverbs 23:7 says, “For as he calculates in his soul, so is he.”

You want to know what you really are? Take a look at your heart.

This post is based on a sermon Dr. MacArthur preached in 1994, titled “Keeping a Pure Mind.” In addition to serving as the pastor of Grace Community Church and the voice of Grace to You, Dr. MacArthur is the chancellor of The Master’s University in Santa Clarita, Calif. You can learn more about TMU at www.masters.edu.