Divorce
v.31-32 Citing Dt 24:1, Jesus points out the OT law about divorce. He then provides the correct interpretation – marriages are meant to be permanent. See also Mt 19:3-9. God intended man and woman to be joined together as one flesh (Gen 2:24) (i.e., a permanent union). Divorce results in adultery. Jesus allows an exception for marital unfaithfulness.
Lust
v.27-28 Adultery is an offense that violates the 10 commandments (Ex 20:14) and breaks the marriage covenant (Mal 2:14). Jesus interprets it for us by showing that looking at a woman with lustful intent in your heart is the same as having committed adultery.
v.29-30 Even things of great value should be given up if they are causing you to sin. Your right eye or your hand, if they cause you to sin, should be cut off. It is better to go to heaven and be short an eye than to go to hell with two eyes.
In verses 21 to 48 Jesus provides a fuller explanation of the OT law in six examples. They illustrate a righteousness that goes beyond the outward correctness of the Pharisees and scribes.
Anger.
v.21-22 Here Jesus quotes the OT prohibition on murder from Ex 20:13. Murderers were subject to judgment (Nm 35:31). But Jesus teaches that everyone who is angry with his brother (without cause) is also subject to judgment. In this context, “brother” means anyone among Christ’s disciples. The basis is that each person is made in God’s image and we need to value our brother (or sister in Christ) as God values him. It is not only outward murder, but also inward anger (that could lead to murder) that violates God’s prohibition on murder in the Ten Commandments. Where unjustified, Jesus also condemns the use of insulting names.
v.23-24 Reconciliation with a brother should take priority over even offering a gift in worship. This is not one’s own anger but the anger of a brother that needs to be dealt with. Anger between you and a brother can effect your relationship with God and should be dealt with immediately.
v.25-26 To not get reconciliation can have disastrous effects as shown in the example in these verses. Here we are dealing with an adversary who apparently has a monetary claim against you. Settle with him out of court or you may go to prison until you have paid the last penny.
Fulfillment of the Law and Prophets.
v.17 Jesus came to fulfill the law of Moses and the rest of the OT. Jesus’ teaching does not contradict the OT law, but rather unfolds it and transcends it. Christ is the end or goal of the law (Rom 10:4). The law and the prophets point to him. Jesus did not oppose the law, but rather the illegitimate interpretation of it that stressed regulations instead of internal character.
v.18 The law is permanent (not a letter or stroke of the pen will pass away) until all has been accomplished (heaven and earth pass away). Correct interpretation of the OT law must take into account the NT teachings. Jesus gives examples of such NT interpretations of OT law in the coming verses (v.21-48).
v.19 Jesus’ teaching and the OT, properly interpreted, must be obeyed (perfection, v.48). The verse seems to indicate graduations of honor/dishonor within God’s kingdom. Teachers must not only follow the commands, but teach others to do the same.
v.20 The righteousness that the law pointed to is required for admission to the kingdom. (This of course is only possible for us through Christ.) The Pharisees had outward conformity to many regulations but still had impure hearts inside.