No one ever “falls” into adultery. The adulterer’s heart is always shaped and prepared by lustful thoughts before the actual deed occurs. Likewise, the heart of the thief is bent by covetousness. And murder is the product of anger and hatred. All sin is first incubated in the mind.
Jesus taught this truth to His disciples: “The things that proceed out of the mouth come from the heart, and those defile the man. For out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders. These are the things which defile the man; but to eat with unwashed hands does not defile the man” (Matthew 15:18-20, emphasis added).
Jesus was teaching that the real point of the Mosaic law was the moral truth embodied in the external ceremonial requirements. He downplayed the symbolic aspects of washing and abstaining from what is legally declared unclean. Instead He emphasized the moral requirement of the law. Defilement, He suggested, is not primarily a ceremonial or external problem; what is truly defiling in the spiritual sense is the wickedness that emanates from the heart. In Scripture, “the heart” is the seat of the whole person—mind imagination, affections, and will. “Heart” is often used as a synonym for “mind.” In these verses, therefore, our Lord was condemning the wickedness of an impure thought life.
Again and again, Christ rebuked the Pharisees for their fastidious observance of the external, ceremonial law and their wanton neglect of the law’s moral requirements. They were utterly preoccupied with appearing to be righteous. Yet they were willing to tolerate the grossest sins of the heart. They thought no one else could ever discover what was really inside them. But our Lord knew what was in their hearts (Matthew 9:4; 12:25). He compared them to elegant crypts, beautiful on the outside but full of defilement and death on the inside.
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. You blind Pharisee, first clean the inside of the cup and of the dish, so that the outside of it may become clean also. Woe to you, scribes and Pharisees, hypocrites! For you are like whitewashed tombs which on the outside appear beautiful, but inside they are full of dead men’s bones and all uncleanness. So you, too, outwardly appear righteous to men, but inwardly you are full of hypocrisy and lawlessness (Matthew 23:25-28).
The Pharisees’ teaching had so indoctrinated this notion into people that it was commonly believed evil thoughts were not really sinful, as long as they did not become acts. That is precisely why our Lord targeted sins of the heart in His Sermon on the Mount.
You have heard that the ancients were told, “You shall not commit murder” and “Whoever commits murder shall be liable to the court.” But I say to you that everyone who is angry with his brother shall be guilty before the court. . . . You have heard that it was said, “You shall not commit adultery”; but I say to you that everyone who looks on a woman with lust for her has already committed adultery with her in his heart (Matthew 5:21-22, 27-28).
What should take place in our minds and hearts? What should be the deepest secret of our souls? Worship to God.
When you give to the poor, do not let your left hand know what your right hand is doing, so that your giving will be in secret; and your Father who sees what is done in secret will reward you. When you pray, you are not to be like the hypocrites; for they love to stand and pray in the synagogues and on the street corners, so that they may be seen by men. Truly I say to you, they have their reward in full. But you, when you pray, go into your inner room, close your door and pray to your Father who is in secret, and your Father who sees what is done in secret will reward you (Matthew 6:3-6).
To sin in the mind, therefore, is to desecrate the very sanctuary where our highest and best worship should be taking place.
Regardless of how you present yourself to the outside world, the Lord has a front-row seat to everything that goes on in your mind. Are you glorifying Him with your thoughts? Or are you cultivating sinful mental habits that will cripple your spiritual growth and poison your heart? The patterns of your thought life are borne out in your behavior—what does yours say about you?
Next week we’ll look at some practical ways from Scripture to guard against evil thoughts and to discipline your mind.