St. Thomas Aquinas clearly explained that law was divided into moral law, ceremonial law, and judicial law. Moral law still exists in the New Covenant because of the clear NT passages that show the commandments still hold. Also, moral law was from the beginning, deriving from the law of nature. Therefore, as long as nature, or creation, exists, it is binded to moral law. The Catechism teaches:
“2068 The Council of Trent teaches that the Ten Commandments are obligatory for Christians and that the justified man is still bound to keep them; the Second Vatican Council confirms: 'The bishops, successors of the apostles, receive from the Lord … the mission of teaching all peoples, and of preaching the Gospel to every creature, so that all men may attain salvation through faith, Baptism and the observance of the Commandments.”
- "The Ten Commandments belong to God’s revelation. At the same time they teach us the true humanity of man. They bring to light the essential duties, and therefore, indirectly, the fundamental rights inherent in the nature of the human person. The Decalogue contains a privileged expression of the natural law: “From the beginning, God had implanted in the heart of man the precepts of the natural law. Then he was content to remind him of them. This was the Decalogue” (St. Irenaeus, Adv. haeres. 4, 15, 1: PG 7/1, 1012).
- “Since they express man’s fundamental duties towards God and towards his neighbour, the Ten Commandments reveal, in their primordial content, grave obligations. They are fundamentally immutable, and they oblige always and everywhere. No one can dispense from them. The Ten Commandments are engraved by God in the human heart.”