Blessed are they who follow the law of the Lord. The Responsorial Psalm for today celebrates and thanks God for the Torah, the law. The Psalmist praises God for giving such splendid laws and instruction for people to live by. He then begs to be granted the wisdom to understand and keep the precepts of the law since, as Sirach reminds us in the First Reading, choosing to keep the law of the Lord gives life. Echoing Moses’ last instruction to the Israelites (cf. Deuteronomy 30:15ff), Sirach reminds us that whereas one is at liberty to do as he/she wills (free will), it is always wise to choose to abide by the law of the Lord. He/she who chooses to abide by the law of the Lord chooses goodness and life. Such a person, says Sirach, is looked upon favorably by the Lord. Perhaps this is why we hear Jesus in the Gospel Reading asserting that he came, not to make obsolete but rather to fulfill the law. 
Our reflection on the Sermon on the Mount continues today as we see Jesus addressing an issue that was of central importance to his community (or rather to the majority of those who had gathered to listen to him). The law was central to the Jewish people because their lives revolved around it. Every single aspect of their lives was governed by, and revolved around the law: the law prescribed how an animal was to be slaughtered, how food was to be prepared and served, how the seats were to be arranged both at home and in places of worship, and how to wash one’s hands before eating. Naturally, as it happens wherever there is a law, there must have been those who protested such encroachment by law into their personal spaces. Just like is the case today, such individuals might have believed that with time, the law gets to outlive its usefulness and as such must be reviewed if not completely overhauled. Such individuals might have even gotten emboldened by Jesus’ proclamation of a new dispensation ( you have heard it said…but I say to you...) 
Jesus had been in the scene for not so long a time and had already performed some mighty signs on account of which the people had begun to talk. Because of the signs, many were already endearing themselves to Jesus, a fact that did not go well with everybody, especially those who were seeing in Jesus a liberal approach to matters of religion and morality (he would later be accused of communing with sinners and outcasts [cf. Matthew 9:10-13]). Those who had been marginalized saw in Jesus a champion, someone who had come to uphold their dignity and fight for their cause. It might as well have happened that even those who were indifferent to the law and to any form of regulation (libertarians) began to point to Jesus as their vindication. Such individuals might have gone as far as stating that the law had become obsolete as they pointed to the fact that Jesus did not observe the law and tradition with the same scrupulous severity as the religious leaders yet God was working mighty signs through him. It is perhaps to those who might have harbored such thoughts that Jesus directs his assertion that he has come not to abolish but rather to fulfill the law. His assertion is a categorical statement that he neither represents nor shares in such thoughts. Not only does Jesus affirm the relevance of the law but also associates the success of his mission with the fact that he upholds the law. So what does Jesus mean when he says he is the fulfillment of the law?
While it is true that Jesus’ mission was not the abolishing of the law, his approach to, and understanding of the law was much different from that of contemporary religious leaders. Using the formula ‘ you have heard it said…but I say to you…,’ Jesus points to the fact that there was something lacking in the existing implementation of the law, a lack that he had come to fill. For Jesus, the law as it existed laid too much emphasis on the letter to the detriment of the spirit. With the spirit of the law tossed aside, it became too easy for an individual to transgress against the law without realizing it. The law as it existed focused on executed acts (such as murder) while neglecting the circumstances that leads to murder itself (such as anger). To Jesus, an individual who plots the murder of another is guilty of the offence even if the execution of the act does not take place. This is because the person who plots or mulls over in his/her head the murder of another has already broken the bond of charity that ties him/her to the one he/she plans to murder. In his/her heart, the individual is already dead. 
The fulfillment that Jesus brings to the existing law is meant to give life to the law, that is, to help the law live to its billing of preserving relationships. The law must not only deal with the external but with the internal as well. The law must extend to, and speak to an individual’s heart for the heart is the core of a person and the source of decision and action. Any transgression against the law (sin) is a transgression against the relationships into which God has inserted us. These relationships are nurtured internally in the depths of our hearts. To preserve and thrive in a relationships, then, one must retreat to his/her heart and begin from there. This is the whole point of the law and it is to this that Jesus want us to focus.