"Against the Heresies" by St. Irenaeus
In the book of Deuteronomy Moses says to the people: "The Lord our God made a covenant with us at Horeb. It was not with our fathers that the Lord made this covenant, but with us, with us who are here, all living today" (Deuteronomy 5:2-3). Why did God not make this covenant with their fathers? Because "laws are not framed for people who are good" (1 Timothy 1:9). Their fathers were righteous: they had the power of the Decalogue implanted in their hearts and in their souls. That is, they loved the God who made them and did nothing unjust against their neighbour. For this reason they did not need to be admonished by written rebukes: they had the righteousness of the law in their hearts.
When this righteousness and love for God had passed into oblivion and had been extinguished in Egypt, God had necessarily to reveal himself through his own voice, out of his great love for men. He led the people out of Egypt in power, so that man might once again become God’s disciple and follower. He made them afraid as they listened, to warn them not to hold their Creator in contempt. "He fed them with manna, that they might receive spiritual food. In the book of Deuteronomy Moses says: he fed you with manna which neither you nor your fathers had known, to make you understand that man does not live on bread alone but that man lives on everything that comes from the mouth of the Lord" (Deuteronomy 8:3).
He commanded them to love himself and trained them to practise righteousness towards their neighbour, so that man might not be unrighteous or unworthy of God. Through the Decalogue he prepared man for friendship with himself and for harmony with his neighbour. This was to man’s advantage, though God needed nothing from man.
This raised man to glory, for it gave him what he did not have, friendship with God. But it brought no advantage to God, for God did not need man’s love. Man did not possess the glory of God, nor could he attain it by any other means than through obedience to God. This is why Moses said to the people: "Choose life, then, so that you and your descendants may live, in the love of the Lord your God, obeying his voice, clinging to him; for in this your life consists, and on this depends your long stay in the land which the Lord swore to your fathers Abraham, Isaac and Jacob he would give" (Deuteronomy 30:20).
This was the life that the Lord was preparing man to receive when he spoke in person and gave the words of the Decalogue for all alike to hear. These words remain with us as well; they were extended and amplified through his coming in the flesh, but not annulled.
God gave to the people separately through Moses the commandments that enslave: these were precepts suited to their instruction or their condemnation. As Moses said: "the Lord ordered me then to teach you the laws and customs that you were to observe" (Deuteronomy 4:14). The precepts that were given them to enslave and to serve as a warning have been cancelled by the new covenant of freedom. The precepts that belong to man’s nature and to freedom and to all alike have been enlarged and broadened. Through the adoption of sons God has enabled man so generously and bountifully to know him as Father, to love him with his whole heart, and to follow his Word unfailingly.