As Jesus applies the law of God in this sermon, He continues the same pattern. Obedience to the law is not merely a matter of external conformity; it is a question of heart loyalty. Put another way, the well-respected and pious tend to think about sins. The godly are constrained by the teaching of Christ to think about sin -- the condition of sinfulness, the fountainhead of all sins.
"You have heard that it was said to those of old, ‘"You shall not commit adultery.’" "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart. "If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell. And if your right hand causes you to sin, cut it off and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell (Matt. 5:27-30).
The problem is heart rebellion. Jesus applies the law of God in a way which "decent" people do not like. His application makes all men adulterers. His teaching is plain. We see this principle elsewhere when Christ taught about money. "And He said to them, "You are those who justify yourselves before men, but God knows your hearts. For what is highly esteemed among men is an abomination in the sight of God" (Luke 16:15). What a fearful thought! God knows the heart. And what does He see there? ""For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies" (Matt. 15:19; cf. Mark 7:21). The source of sins is sin. The source of sins is the human heart.
So what is the horror of sin? Christ does not just say in this passage that real adultery is a matter of the heart. He says that such inner adultery is of immense importance to the sinner. How important is it?Rather than fall under the judgment of God for sin, the sinner should prefer self-mutilation. If removing eyes and hands would remove sin, then sin is serious enough to do exactly that. Because the context of this passage is the teaching about adultery on one side and then divorce on the other, I believe that Christ is alluding to the preferability of self-castration to the wrath of God. But there is another problem. Self-mutilation does not work in restraining sin. Neither will asceticism (Col. 2:23).
So what is the offending member that must be cut off? What brings us to sin? What produces sin in us? What must therefore be cut off? The answer is that my offending member is my heart. The doctrine is that I must have a new heart, or I will die.
The solution is therefore heart regeneration. We must get the doctrine of regeneration straight in our minds. A tremendous amount of mischief has resulted from confusion at this point. We are not born again because we have repented and believed. Rather, we have repented and believed because God has given us the new birth. If my old heart was capable of repentance and belief, then I did not need a new heart. I simply needed to have improved the old one. What does God promise through the ancient prophet? "I will give you a new heart and put a new spirit within you; I will take the heart of stone out of your flesh and give you a heart of flesh" (Ez. 36:26). Who will do this? The only one who can. See verse 27.
The result is heart obedience. Jesus calls His followers to a genuine submission to the law of God and Christ. If we cannot, then we must cry out for a new heart. If we are believers who have drifted into compromise, then we must confess our sin to Him. He is faithful and just to forgive. Our new hearts enable us to look to Him for all our righteousness. True purity is found only there.