4 The Lord, too, does not do away with this [God], when He shows that the law was not derived from another God, expressing Himself as follows to those who were being instructed by Him, to the multitude and to His disciples: “The scribes and Pharisees sit in Moses' seat. All, therefore, whatsoever they bid you observe, that observe and do; but do not after their works: for they say, and do not. For they bind heavy burdens, and lay them upon men's shoulders; but they themselves will not so much as move them with a finger.” He therefore did not throw blame upon that law which was given by Moses, when He exhorted it to be observed, Jerusalem being as yet in safety; but He did throw blame upon those persons, because they repeated indeed the words of the law, yet were without love.
And for this reason were they held as being unrighteous as respects God, and as respects their neighbours. As also Isaiah says: “This people honours Me with their lips, but their heart is far from Me: howbeit in vain do they worship Me, teaching the doctrines and the commandments of men.” He does not call the law given by Moses commandments of men, but the traditions of the elders themselves which they had invented, and in upholding which they made the law of God of none effect, and were on this account also not subject to His Word.
For this is what Paul says concerning these men: “For they, being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves to the righteousness of God. For Christ is the end of the law for righteousness to every one that believes.” And how is Christ the end of the law, if He be not also the final cause of it? For He who has brought in the end has Himself also wrought the beginning; and it is He who does Himself say to Moses, “I have surely seen the affliction of my people which is in Egypt, and I have come down to deliver them;” it being customary from the beginning with the Word of God to ascend and descend for the purpose of saving those who were in affliction.
Source: Adversus haereses (New Advent)