What does it mean to lust in your heart (Matthew 5:28)?

The Lord Jesus now explains that before God it is not just the accomplished act that is sin, but the looking with lust on a woman, for this is adultery in the heart. The word "adultery" shows that either the man or both are married. Nevertheless, no unmarried believer should think that these words of the Lord have nothing to say to him.

The Lord is not speaking here about accidental, unintentional looks which can hardly be avoided, but about the conscious covetous looking: "Whosoever looketh on a woman to lust after her..." The intentional look is therefore preceded by the sinful thought in the heart.

The Lord Jesus now explains that before God it is not just the accomplished act that is sin, but the looking with lust on a woman, for this is adultery in the heart. The word “adultery” shows that either the man or both are married. Nevertheless, no unmarried believer should think that these words of the Lord have nothing to say to him.

The Lord is not speaking here about accidental, unintentional looks which can hardly be avoided, but about the conscious covetous looking: “Whosoever looketh on a woman to lust after her … ” The intentional look is therefore preceded by the sinful thought in the heart.

For that reason the Lord now says, "But I say unto you, That whosoever looketh on a woman to lust after her, hath committed adultery with her already in his heart" (v. 28). Because of the teaching of the scribes and Pharisees, the Jews thought the mere outward observance of the law was the way to be justified before God. Here, the Lord points to the human heart and shows that adultery has its source there. This is not a "spiritualisation of the law," as is sometimes said. He reveals for the first time something which had to become clear by experience to every honest Israelite, namely, that everyone who endeavoured to keep the commandment, "Thou shalt not commit adultery," had within themselves those very lusts which led to the actions forbidden by God, and had not the strength to overcome them. The lusts were even provoked by the commandment: "... for I had not known lust, except the law had said, Thou shalt not covet" (Rom. 7: 7-11).

For that reason the Lord now says, “But I say unto you, That whosoever looketh on a woman to lust after her, hath committed adultery with her already in his heart” (v. 28). Because of the teaching of the scribes and Pharisees, the Jews thought the mere outward observance of the law was the way to be justified before God. Here, the Lord points to the human heart and shows that adultery has its source there. This is not a “spiritualisation of the law,” as is sometimes said. He reveals for the first time something which had to become clear by experience to every honest Israelite, namely, that everyone who endeavoured to keep the commandment, “Thou shalt not commit adultery,” had within themselves those very lusts which led to the actions forbidden by God, and had not the strength to overcome them. The lusts were even provoked by the commandment: “ … for I had not known lust, except the law had said, Thou shalt not covet” (Rom. 7:7-11).

Let one instance suffice out of the seven occasions in that discourse when our Lord said, “I say to you.” “Ye have heard that it was said of them of old time, Thou shalt not commit adultery: but I say to you, That whosoever looks on a woman to lust after her has committed adultery with her already in his heart” (Matt. 5:27-28). Where is there the setting aside of the law? Nay, He upheld it. The law was content to forbid the actual act, the Lord went further, and not only forbad the outward act, but the inward desire.

As violence then was judged and excluded in any shape for the disciples, so was impurity. The avoidance of the extreme act might satisfy a Pharisee or Scribe; but the Lord could not dispense with anything short of truth in the inward parts. To look at a woman lustfully was to commit adultery with her already in his heart; and it is not the outside only that God regards but the heart above all. It is only a new nature that delights in holiness; and he who has it by grace answers to the will of God his Father; and abhors himself if he slip even into a wrong look, as unworthy of his calling and hateful to Him who loves him.

The next two expansions of the law are concerning purity and divorce. Not alone the deed itself, which was punishable by a severe penalty, is sin, but every one who looks upon a woman to lust after her has already committed adultery with her in his heart. Out of the heart the transgression comes, and the heart is evil. And this is what so many persons in Christendom say is their creed and standard for "religion"! It is the word which condemns them altogether.

The question of inner thoughts is again solemnly pressed in verses 27 to 29. Though the act of adultery may not be outwardly perpetrated, yet a lusting heart is guilty Of this. This is of course a question of God's judgment, not men's, for men's public government con only judge when evil becomes manifest through action. The Lord is here seeking to reach individual consciences, that men may judge themselves. The right eye is ideally the eye of faith, typically speaking, as the left eye is that of reason. If faith fails in any way, let us judge this unsparingly, for what one sees can easily do damage to his faith if he does not honestly judge it. In fact, One who never judges himself does not have faith at all (Mk.9:43-46), in Which Case he can expect only the fire of hell.

It is sometimes said that deliverance from sin is a matter of personal effort, and that evil habits can and should be overcome by one's own firm resolution and perseverance. But this advice overlooks the fact that although the wicked deed may be checked by will-power alone, the sinful desire fills the heart beforehand. And divine holiness uncovers the secret source of inward evil. In God's sight, the thought of foolishness is sin. Our Lord condemns alike the lustful look and the adulterous act (Matt. 5:28). The evil thought defiles the whole man, as well as the evil word and the evil act (Mark 7:21-23). We should habitually govern our words and our deeds, but who can govern the imagination of the heart which God beholds? If we are walking with God, we are with One Who knows our thoughts even before we recognise them ourselves. The holiness of God is very pure, infinitely above any human standard, and in order to act rightly with regard to that standard, we must attend to the words of scripture concerning the “sin which doth so easily beset us.”

It has been urged by some that the Lord's words in verses 21, 27, 33, 43, "Ye have heard it said of them of old time… but l say to you, " give a warrant for discarding the Scriptures and substituting for them something up to date. It is a scandalous and immoral procedure. He was shifting the searchlight from the outward actions to the unseen inner nature. One reason for which the law was given was to check and curb the lusts and passions of man, to hold within bounds the evil acts of his fallen nature. This is shown in Paul's first letter to Timothy. "But we know that the law is good if a man use it lawfully: knowing this, that the law is not made for a righteous man, but for the lawless and disobedient, for the ungodly and sinners, for unholy and profane, for murderers of fathers and murderers of mothers, for man-slayers" etc. (chap. 1:9, 19). Thus had it been rightly interpreted, but the Lord went deeper and dealt with the hidden desires of the heart, with the nature that lies at the root of every transgression and from which all evil deeds spring, and declared that God required truth in the inward parts. David learned in his great repentance, that more terrible than his deeds was the nature that produced them, and the law taught Saul of Tarsus the same necessary lesson (Romans 7). This did not make void the law, it established its use as a searching, convicting power.