Today’s Catechism sections discuss the Ten Commandments and the natural law. Supporting material comes from St. Irenaeus’ “Against Heresies”.

The Decalogue and the natural law

2070 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.31

 2071 The commandments of the Decalogue, although accessible to reason alone, have been revealed. To attain a complete and certain understanding of the requirements of the natural law, sinful humanity needed this revelation:

A full explanation of the commandments of the Decalogue became necessary in the state of sin because the light of reason was obscured and the will had gone astray.32

We know God's commandments through the divine revelation proposed to us in the Church, and through the voice of moral conscience. The obligation of the Decalogue

2072 Since they express man's fundamental duties towards God and towards his neighbor, 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.

2073 Obedience to the Commandments also implies obligations in matter which is, in itself, light. Thus abusive language is forbidden by the fifth commandment, but would be a grave offense only as a result of circumstances or the offender's intention. "Apart from me you can do nothing"

2074 Jesus says: "I am the vine, you are the branches. He who abides in me, and I in him, he it is that bears much fruit, for apart from me you can do nothing."33 The fruit referred to in this saying is the holiness of a life made fruitful by union with Christ. When we believe in Jesus Christ, partake of his mysteries, and keep his commandments, the Savior himself comes to love, in us, his Father and his brethren, our Father and our brethren. His person becomes, through the Spirit, the living and interior rule of our activity. "This is my commandment, that you love one another as I have loved you."34

IN BRIEF

2080 The Decalogue contains a privileged expression of the natural law. It is made known to us by divine revelation and by human reason.

2081 The Ten Commandments, in their fundamental content, state grave obligations. However, obedience to these precepts also implies obligations in matter which is, in itself, light.

2082 What God commands he makes possible by his grace.

In “Against Heresies”, St. Irenaeus discusses the Ten Commandments and natural law.

1. They (the Jews) had therefore a law, a course of discipline, and a prophecy of future things. For God at the first, indeed, warning them by means of natural precepts, which from the beginning He had implanted in mankind, that is, by means of the Decalogue (which, if any one does not observe, he has no salvation), did then demand nothing more of them. As Moses says in Deuteronomy, These are all the words which the Lord spoke to the whole assembly of the sons of Israel on the mount, and He added no more; and He wrote them on two tables of stone, and gave them to me. Deuteronomy 5:22 For this reason [He did so], that they who are willing to follow Him might keep these commandments. But when they turned themselves to make a calf, and had gone back in their minds to Egypt, desiring to be slaves instead of free-men, they were placed for the future in a state of servitude suited to their wish—[a slavery] which did not indeed cut them off from God, but subjected them to the yoke of bondage; as Ezekiel the prophet, when stating the reasons for the giving of such a law, declares: And their eyes were after the desire of their heart; and I gave them statutes that were not good, and judgments in which they shall not live. Ezekiel 20:24 Luke also has recorded that Stephen, who was the first elected into the diaconate by the apostles, and who was the first slain for the testimony of Christ, spoke regarding Moses as follows: This man did indeed receive the commandments of the living God to give to us, whom your fathers would not obey, but thrust [Him from them], and in their hearts turned back again into Egypt, saying unto Aaron, Make us gods to go before us; for we do not know what has happened to [this] Moses, who led us from the land of Egypt. And they made a calf in those days, and offered sacrifices to the idol, and were rejoicing in the works of their own hands. But God turned, and gave them up to worship the hosts of heaven; as it is written in the book of the prophets: Amos 5:25-26 O you house of Israel, have you offered to Me sacrifices and oblations for forty years in the wilderness? And you took up the tabernacle of Moloch, and the star of the god Remphan, figures which you made to worship them; Acts 7:38, etc. pointing out plainly, that the law being such, was not given to them by another God, but that, adapted to their condition of servitude, [it originated] from the very same [God as we worship]. Wherefore also He says to Moses in Exodus: I will send forth My angel before you; for I will not go up with you, because you are a stiff-necked people. Exodus 33:2-3

Footnotes

31 St. Irenaeus, Adv. haeres. 4, 15, 1: PG 7/l, 1012.
32 St. Bonaventure, Comm. sent. 4, 37, 1, 3.
33 ⇒ Jn 15:5.
34 ⇒ Jn 15:12.