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 man fashioned in the image of God; they prohibit what is contrary to the love of God and neighbor 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.
Source: Catechism of the Catholic Church (Vatican.va)