Today's two Mass readings offer contrasting views of the law. Paul says in 2 Cor that the new covenant is not one of law but of Spirit--the written law kills but the Spirit gives life. In the Gospel, Jesus in the Sermon on the Mount says he has not come to abolish the law but to fulfill it.
It seems contradictory but knowing the context helps us sort it out.
In recent years, Scripture scholars have been studying Paul's view of the law more deeply. He did not see it in terms of the polemics that developed during and after the Reformation between Protestants and Catholics. Instead, he was fighting those who would have his Gentile converts adopt the Mosaic law. But Paul had no quarrel with Jewish Christians who wanted to keep on observing the law, as long as they understood that the law didn't justify them. Only the blood of Jesus justifies us. Paul himself observed some precepts of the Jewish law, as when he had Timothy circumcised and when he took the Nazirite vow.
Matthew's Gospel was written for a community of Jewish converts to Christianity. Matthew presented Jesus to them as the fulfillment of everything the law had promised. They knew that their salvation did not come from simply observing the precepts, but through faith in Jesus. But that faith didn't nullify God's covenant with them.