3 And what does this mean my brethren? For it is right for us to investigate the saying of the prophet, and especially on account of heretics who have turned their mind against the law. By Moses then, God gave commandment respecting sacrifices, and all the book called Leviticus is entirely taken up with the arrangement of these matters, so that He might accept the offerer. So through the Prophets, He blames him who despised these things, as disobedient to the commandment saying, 'I have not required these at your hands.
Neither did I speak to your fathers respecting sacrifices, nor command them concerning whole burnt-offerings. ' Now it is the opinion of some, that the Scriptures do not agree together, or that God, Who gave the commandment, is false. But there is no disagreement whatever, far from it, neither can the Father, Who is truth, lie; 'for it is impossible that God should lie,' as Paul affirms. But all these things are plain to those who rightly consider them, and to those who receive with faith the writings of the law.
Now it appears to me— may God grant, by your prayers, that the remarks I presume to make may not be far from the truth— that not at first were the commandment and the law concerning sacrifices, neither did the mind of God, Who gave the law, regard whole burnt-offerings, but those things which were pointed out and prefigured by them. 'For the law contained a shadow of good things to come.' And, 'Those things were appointed until the time of reformation. '
Source: Letters (New Advent)