1 Moreover, the prophets indicate in the fullest manner that God stood in no need of their slavish obedience, but that it was upon their own account that He enjoined certain observances in the law. And again, that God needed not their oblation, but [merely demanded it], on account of man himself who offers it, the Lord taught distinctly, as I have pointed out. For when He perceived them neglecting righteousness, and abstaining from the love of God, and imagining that God was to be propitiated by sacrifices and the other typical observances, Samuel did even thus speak to them: “God does not desire whole burnt-offerings and sacrifices, but He will have His voice to be hearkened to.
Behold, a ready obedience is better than sacrifice, and to hearken than the fat of rams.” David also says: “Sacrifice and oblation You did not desire, but my ears have You perfected; burnt-offerings also for sin You have not required.” He thus teaches them that God desires obedience, which renders them secure, rather than sacrifices and holocausts, which avail them nothing towards righteousness; and [by this declaration] he prophesies the new covenant at the same time. Still clearer, too, does he speak of these things in the fiftieth Psalm: “For if You had desired sacrifice, then would I have given it: You will not delight in burnt-offerings.
The sacrifice of God is a broken spirit; a broken and contrite heart the Lord will not despise.” Because, therefore, God stands in need of nothing, He declares in the preceding Psalm: “I will take no calves out of your house, nor he-goats out of your fold. For Mine are all the beasts of the earth, the herds and the oxen on the mountains: I know all the fowls of heaven, and the various tribes of the field are Mine. If I were hungry, I would not tell you: for the world is Mine, and the fullness thereof.
Shall I eat the flesh of bulls, or drink the blood of goats?” Then, lest it might be supposed that He refused these things in His anger, He continues, giving him (man) counsel: “Offer unto God the sacrifice of praise, and pay your vows to the Most High; and call upon Me in the day of your trouble, and I will deliver you, and you shall glorify Me;” rejecting, indeed, those things by which sinners imagined they could propitiate God, and showing that He does Himself stand in need of nothing; but He exhorts and advises them to those things by which man is justified and draws near to God.
This same declaration does Esaias make: “To what purpose is the multitude of your sacrifices unto Me? Says the Lord. I am full.” And when He had repudiated holocausts, and sacrifices, and oblations, as likewise the new moons, and the sabbaths, and the festivals, and all the rest of the services accompanying these, He continues, exhorting them to what pertained to salvation: “Wash you, make you clean, take away wickedness from your hearts from before my eyes: cease from your evil ways, learn to do well, seek judgment, relieve the oppressed, judge the fatherless, plead for the widow; and come, let us reason together, says the Lord.”
Source: Adversus haereses (New Advent)