Jesus now moves on in His sermon to describe the difference in the actions due to a sense of duty and actions that exist due to a changed heart. In the rest of this chapter we see that all sin originates in the mind and heart of man. Each action that we take has been peculating in our minds and hearts long before the evidence of those thoughts comes to to light in our actions.
Jesus says that if a man looks after a woman to lust after her in his heart he has already committed adultery with her. We see a higher standard than the law gave in the ten commandments. Now we are to guard our hearts and minds. The only way to refrain from sinful actions is to have Jesus change our hearts. The term heart means “the seat of the will.” if we do not will something to happen, it has no seed of intention. But in our fallen nature, we tend to sow seeds that are contrary to God’s commands.
Jesus then continues on and speaks on divorce and shows that a simple certificate of divorce does not condone it. He then says that Moses gave them this (certificate of divorce) due to the hardness of their hearts. If a man left a woman and did not giver her a proper divorce, she would have been unable to care for herself. Women could not own property or earn a living. So Jesus was protecting the innocent, not condoning the sinner.
Jesus then goes on to talk about, among other things, murder. Murder begins with anger. The seed of anger creates a murder to happen. Imagine trying to grow a crop without any seed. It would be impossible to do so. In the same way, in order for us to keep ourselves away from being murderers and slanderers we need to abstain from holding grudges and we are to love one another as Christ first loved us.
Jesus, the fulfillment of the Law, now shows how He fulfills it. We no longer strive to “do different” we instead mourn over our sin, repent and Jesus creates in us the ability to “be different.” Only when we have had that change in our lives are we truly the children of God.