3 Why, then, did the Lord not form the covenant for the fathers? Because “the law was not established for righteous men.” But the righteous fathers had the meaning of the Decalogue written in their hearts and souls, that is, they loved the God who made them, and did no injury to their neighbour. There was therefore no occasion that they should be cautioned by prohibitory mandates (correptoriis literis), because they had the righteousness of the law in themselves.
But when this righteousness and love to God had passed into oblivion, and became extinct in Egypt, God did necessarily, because of His great goodwill to men, reveal Himself by a voice, and led the people with power out of Egypt, in order that man might again become the disciple and follower of God; and He afflicted those who were disobedient, that they should not contemn their Creator; and He fed them with manna, that they might receive food for their souls (uti rationalem acciperent escam); as also Moses says in Deuteronomy: “And fed you with manna, which your fathers did not know, that you might know that man does not live by bread alone; but by every word of God proceeding out of His mouth does man live.” And it enjoined love to God, and taught just dealing towards our neighbour, that we should neither be unjust nor unworthy of God, who prepares man for His friendship through the medium of the Decalogue, and likewise for agreement with his neighbour—matters which did certainly profit man himself; God, however, standing in no need of anything from man.
Source: Adversus haereses (New Advent)