7 And now after threatening the transgressors, and setting great rewards for them that do right, and signifying that He justly requires of us something beyond the former measures; He from this point begins to legislate, not simply, but by way of comparison with the ancient ordinances, desiring to intimate these two things: first, that not as contending with the former, but rather in great harmony with them, He is making these enactments; next, that it was meet and very seasonable for Him to add thereto these second precepts.
And that this may be made yet clearer, let us hearken to the words of the Legislator. What then does He Himself say?
“You have heard that it was said to them of old time, You shall not kill.”
And yet it was Himself who gave those laws also, but so far He states them impersonally. For if on the one hand He had said, “You have heard that I said to them of old,” the saying would have been hard to receive, and would have stood in the way of all the hearers. If again, on the other hand, after having said, “You have heard that it was said to them of old by my Father,” He had added, “But I say,” He would have seemed to be taking yet more on Himself.
Wherefore He has simply stated it, making out thereby one point only; the proof that in fitting season He had come saying these things. For by the words, “It was said to them of old,” He pointed out the length of the time, since they received this commandment. And this He did to shame the hearer, shrinking from the advance to the higher class of His commandments; as though a teacher should say to a child that was indolent, “Do you not know how long a time you have consumed in learning syllables?” This then He also covertly intimates by the expression, “them of old time,” and thus for the future summons them on to the higher order of His instructions: as if He had said, “You are learning these lessons long enough, and you must henceforth press on to such as are higher than these.”
And it is well that He does not disturb the order of the commandments, but begins first with that which comes earlier, with which the law also began. Yea, for this too suits with one showing the harmony between them.
“But I say unto you, that whosoever is angry with his brother without a cause, shall be in danger of the judgment.”
Do you see authority in perfection? Do you see a bearing suited to a legislator? Why, which among prophets ever spoke on this wise? Which among righteous men? Which among patriarchs? None; but, “Thus says the Lord.” But the Son not so. Because they were publishing their Master's commands, He His Father's. And when I say, “His Father's,” I mean His own. “For mine,” says He, “are yours, and yours are mine.” And they had their fellow-servants to legislate for, He His own servants.
Let us now ask those who reject the law, “is, 'Be not angry' contrary to 'Do no murder'? Or is not the one commandment the completion and the development of the other?” Clearly the one is the fulfilling of the other, and that is greater on this very account. Since he who is not stirred up to anger, will much more refrain from murder; and he who bridles wrath will much more keep his hands to himself. For wrath is the root of murder. And you see that He who cuts up the root will much more remove the branches; or rather, will not permit them so much as to shoot out at all. Not therefore to abolish the law did He make these enactments, but for the more complete observation of it. For with what design did the law enjoin these things? Was it not, that no one might slay his neighbor? It follows, that he who was opposing the law would have to enjoin murder. For to murder, were the contrary to doing no murder. But if He does not suffer one even to be angry, the mind of the law is established by Him more completely. For he that studies to avoid murder will not refrain from it equally with him that has put away even anger; this latter being further removed from the crime.
Source: Homilies on the Gospel of St. Matthew (New Advent)