In Deuteronomy Moses says to the people, The Lord Your God made a covenant with you in Horeb, not with your fathers did the Lord make this covenant but with you.
Why did the Lord not make the covenant with your fathers? Because The Law is not laid down for the just.
Your fathers lived just lives because they had the meaning of the decalogue implanted in their hearts and minds – that is, they loved God, who made them, and they did their neighbour no injury.
So they did not need to be warned by written prohibitions; for they had the righteousness of the Law in their hearts.
When, however, in Egypt this righteousness and this love towards God were forgotten and became extinct, God was compelled by his deep love towards men to reveal himself by a voice.
With power he led his people out of Egypt, so that man again might become the disciple of God and follow him. So that they might not despise their creator, he punished those who were disobedient. He fed them with manna so that they might have spiritual food.
[…] He taught them to love God, and instilled in them that righteousness which is towards their neighbour. By the Decalogue he instructed men to be friends with himself and in harmony with their neighbour.
Man is greatly helped by these things. God, however, stands in need of nothing from man.These blessings made man glorious, giving him what he lacked: friendship with God. They bestowed nothing on God, for God did not stand in need of man’s love.
Man did not have the glory of God. The only way that man could receive this glory was by obeying God. Therefore Moses said, Choose life that you and your descendants may live, loving the Lord your God and obeying his voice and cleaving to him; for that means life to you and length of days.
To prepare man for this life, God himself spoke the words of the Decalogue, to all men alike. And so these words remain with us too.
[…] By the new covenant of liberty God cancelled those provisions which he had given to his people to enslave them and serve the purpose of a sign. At the same time the laws, which are natural and appropriate to free men and are applicable to all without distinction, were amplified and widened.
Out of the abundance of his love, without grudging, God adopted men as his sons, and granted that they might know God as Father and love him with all their heart, and follow his Word without turning aside.
Irenaeus of Lyons (2nd century AD – c. 202): Adversus Haereses 4.16.2-5); from the Monastic Office of Vigils, Saturday after Ash Wednesday, Year 1.