"I have come not to abolish the Law but to fulfill it"
God, our Creator and Redeemer, chose Israel for himself to his people and revealed his Law to them, thus preparing for the coming of Christ. The Law of Moses expresses many truths naturally accessible to reason. These are stated and authenticated within the covenant of salvation. The Old Law is the first stage of revealed law. Its moral prescriptions are summed up in the Ten Commandments. The precepts of the Decalogue lay the foundations for the vocation of fashioned in the image of God; they prohibit what is contrary to the love of God and neighbour, and prescribe what is essential to it. The Decalogue is a light offered to the conscience of every man to make God's call and ways known to him, and to protect him against evil: “God wrote on the tables of the Law what men did not read in their hearts” (St. Augustine).
According to Christian tradition, the law is holy, spiritual and good, yet still imperfect (Rom 7,12). Like a tutor (Gal 3,24) it shows what must be done, but does not of itself give the strength, the grace of the Spirit, to fulfil it. Because of sin, which it cannot remove, it remains a law of bondage... It is a preparation for the Gospel.
The New Law, or Law of the Gospel is the perfection here on earth of the divine law, natural and revealed. It is the work of Christ and is expressed particularly in the Sermon on the 581 Mount. It is also the work of the Holy Spirit and through him it becomes the interior law of charity: 'I will establish a New Covenant with the house of Israel. .. I will put my laws into their minds, and write them on their hearts, and I will be their God, and they shall be my people” (Heb 8,8-10)
The New Law is the grace of the Holy Spirit given to the faithful through faith in Christ... It “fulfils”, refines, surpasses and leads the Old Law to its perfection. In the Beatitudes (Mt 5,3f.), the New Law fulfils the divine promises by elevating and orienting them toward the 'kingdom of heaven' . It is addressed to those open to accepting this new hope with faith - the poor, the humble, the afflicted, the pure of heart, those persecuted on account of Christ - and so marks out the surprising ways of the Kingdom.