Read Matthew 5:17-20, Psalm 40:6-7, and Jeremiah 23:6
Yesterday, we saw that Christ fulfills the Old Testament promise of a prophet. Christ also fulfills the Old Testament promise of a priest. We also said that the way in which he fulfills these promises can sometimes disappoint and even offend.
So what does Jesus do as a priest? Jeremiah prophesied that this priest would be called the "Lord Our Righteousness" (Jer. 23:6), and the Psalmist explains that he would become our righteousness by actively fulfilling the requirements of the law for us. This is what he did, for instance, when he was baptized for us. Jesus told John that he must be baptized to fulfill all righteousness (Mt. 3:15).
The surprise of legalism, however, is that it is easier than living by grace.
So how could this possibly be a disappointment to us? It is because it is the opposite of legalism, which believes that one must become his own righteousness. In other words, it is the thought that we earn God's approval or increase his approval by doing good things. Deep down, we all like to "earn our keep" by our own actions.
The surprise of legalism, however, is that it is easier than living by grace. If one is going to stick with legalism very long, then they have to redefine the rules or they will give up. That is what these religious leaders were doing. To keep the law of adultery, they had to reduce it to a prohibition of physical involvement only. To keep the law of helping one's neighbor, they had to redefine their neighbor as the one they wanted to help. Long-term legalism is easy. You first pick what you think are the important laws. If you are an orderly person, then for you the laws all have to do with order. If you like certain forms of worship, then the most important laws are those expressing your views. If you are a social activist, then the laws of mercy are most important. Then you define how those things are to be carried out, which often happens to be the way one mostly does it. Keeping these laws inflates our pride and causes us to judge others.
However, to live by Christ's righteousness is at once freer and harder. When you fully acknowledge that he is your righteousness, then you are free to look at all of the laws and say, "I must do them out of gratitude because he has made me able to do them. I will earn no more favor because he has already fulfilled the law for me. But he has also fully equipped me to do these, and by doing them I prove, I am his and that I love him." Therefore, you look at the law of love and see that it not only commands you to love your friends, but also your enemies. Then you look at the law against coveting and realize it actually demands that you rejoice when someone else is better off than you are.
In other words, the law holds up a standard that is beyond anyone's imagination or ability. Yet as the Christian embraces the command, he finds the enablement of Christ's righteousness granted with it. And such enablement surpasses any virtue he could ever conjure up on his own and thus the righteousness of Pharisees and legalists.
The incarnation is "good news of great joy" because it means that Jesus came, fulfilled all righteousness, imputes it to us through faith, and frees us from the futile life of legalism.