4 Therefore, the whole law did not treat of sacrifices, though there was in the law a commandment concerning sacrifices, that by means of them it might begin to instruct men and might withdraw them from idols, and bring them near to God, teaching them for that present time. Therefore neither at the beginning, when God brought the people out of Egypt, did He command them concerning sacrifices or whole burnt-offerings, nor even when they came to mount Sinai. For God is not as man, that He should be careful about these things beforehand; but His commandment was given, that they might know Him Who is truly God, and His Word, and might despise those which are falsely called gods, which are not, but appear in outward show.
So He made Himself known to them in that He brought them out of Egypt, and caused them to pass through the Red Sea. But when they chose to serve Baal, and dared to offer sacrifices to those that have no existence, and forgot the miracles which were wrought in their behalf in Egypt, and thought of returning there again; then indeed, after the law, that commandment concerning sacrifices was ordained as law; so that with their mind, which at one time had meditated on those which are not, they might turn to Him Who is truly God, and learn not, in the first place, to sacrifice, but to turn away their faces from idols, and conform to what God commanded.
For when He says, 'I have not spoken concerning sacrifices, neither given commandment concerning whole burnt-offerings,' He immediately adds, 'But this is the thing which I commanded them, saying, Obey My voice, and I will be to you a God, and you shall be to Me a people, and you shall walk in all the ways that I command you.' Thus then, being before instructed and taught, they learned not to do service to any one but the Lord. They attained to know what time the shadow should last, and not to forget the time that was at hand, in which no longer should the bullock of the herd be a sacrifice to God, nor the ram of the flock, nor the he-goat, but all these things should be fulfilled in a purely spiritual manner, and by constant prayer, and upright conversation, with godly words; as David sings, 'May my meditation be pleasing to Him.
Let my prayer be set forth before You as incense, and the lifting up of my hands as the evening sacrifice. ' The Spirit also, who is in him, commands, saying, 'Offer unto God the sacrifice of praise, and pay to the Lord your vows. Offer the sacrifice of righteousness, and put your trust in the Lord. '
Source: Letters (New Advent)