Matt. V. 17.
“Think not that I have come to destroy the Law or the Prophets.”
Why, who suspected this? Or who accused Him, that He should make a defense against this charge? Since surely from what had gone before no such suspicion was generated. For to command men to be meek, and gentle, and merciful, and pure in heart, and to strive for righteousness, indicated no such design, but rather altogether the contrary.
Wherefore then can He have said this? Not at random, nor vainly: but inasmuch as He was proceeding to ordain commandments greater than those of old, saying, “It was said to them of old time, You shall not kill; but I say unto you, Be not even angry;” and to mark out a way for a kind of divine and heavenly conversation; in order that the strangeness thereof might not disturb the souls of the hearers, nor dispose them quite to mutiny against what He said He used this means of setting them right beforehand.
For although they fulfilled not the law, yet nevertheless they were possessed with much conscientious regard to it; and while they were annulling it every day by their deeds, the letters thereof they would have remain unmoved, and that no one should add anything more to them. Or rather, they bore with their rulers adding thereto, not however for the better, but for the worse. For so they used to set aside the honor due to our parents by additions of their own, and very many others also of the matters enjoined them, they would free themselves of by these unseasonable additions.
Therefore, since Christ in the first place was not of the sacredotal tribe, and next, the things which He was about to introduce were a sort of addition, not however lessening, but enhancing virtue; He knowing beforehand that both these circumstances would trouble them, before He wrote in their mind those wondrous laws, casts out that which was sure to be harboring there. And what was it that was harboring there, and making an obstacle?
Source: Homilies on the Gospel of St. Matthew (New Advent)