Therefore as the breast, when it has fulfilled all its part, and is dismissing the child to the more manly diet, after that appears useless; and the parents who before thought it necessary for the babe, now abuse it with ten thousand mockeries (and many even not content with words of abuse, anoint it also with bitter drugs; that when their words have not power to remove the child's unseasonable propensity towards it, the real things may quench their longing): so also Christ says, that they are of the evil one, not to indicate that the old law is of the devil, but in order that with most exceeding earnestness He might lead them away from their ancient poverty. And to them He says these things; but with regard to the Jews, who were insensible and persevered in the same ways, He has anointed their city all round with the terror of captivity, as with some bitter drug, and made it inaccessible. But since not even this had power to restrain them, but they desired to see it again, running to it, just as a child to the breast, He hid it from them altogether; both pulling it down, and leading away the more part of them far from it: as it is with our cattle; many, by shutting out the calves, in time induce them to forego their old familiar use of the milk.
But if the old law had belonged to the devil, it would not have led people away from idolatry, but rather would have drawn them on and cast them into it; for this did the devil desire. But now we see the opposite effect produced by the old law. And indeed this very thing, the oath, was ordained of old for this cause, that they might not swear by the idols. For “you shall swear,” says He, “by the true God.” They were then no small advantages which the law effected, but rather very great. For that they came unto the “strong meat,” was the work of its care.
“What then,” it may be said, “is not swearing of the evil one?” Yes, indeed it is altogether of the evil one; that is, now, after so high a rule of self-restraint; but then not so.
“But how,” one may say, “should the same thing become at one time good, at another time not good?” Nay, I say the very contrary: how could it help becoming good and not good, while all things are crying aloud, that they are so: the arts, the fruits of the earth, and all things else?
See it, for example, taking place first in our own kind. Thus, to be carried, in the earliest age of life, is good, but afterwards pernicious; to eat food that has been softened in the mouth, in the first scene of our life, is good, but afterwards it is full of disgust; to be fed upon milk and to fly to the breast, is at first profitable and healthful, but tends afterwards to decay and harm. Do you see how the same actions, by reason of the times, appear good, and again not so? Yea, and to wear the robe of a child is well as long as you are a boy, but contrariwise, when you have become a man, it is disgraceful. Would you learn of the contrary case too, how to the child again the things of the man are unsuited? Give the boy a man's robe, and great will be the laughter; and greater the danger, he being often upset in walking after that fashion. Allow him to handle public affairs, and to traffic, and sow, and reap, and great again will be the laughter.
And why do I mention these things? When killing, which among all is acknowledged to be an invention of the evil one, killing, I say, having found its proper occasion, caused Phinehas, who committed it, to be honored with the priesthood. For that killing is a work of him whom I just now mentioned, hear what Christ says; “You will do the works of your Father; he was a manslayer from the beginning.” But Phinehas became a manslayer, and “it was counted unto him” (so He speaks) “for righteousness:” and Abraham again on becoming not a man-slayer only, but (which was far worse) the slayer of his child, won more and more approbation. And Peter too wrought a twofold slaughter, nevertheless what he did was of the Spirit.
Let us not then examine simply the acts, but the season too, and the causes, and the mind, and the difference of persons, and whatsoever else may accompany them, these let us search out with all exactness: for there is no arriving at the truth otherwise.
And let us be diligent, if we would attain unto the kingdom, to show forth something more than the old commandments; since we cannot otherwise lay hold of the things of Heaven. For if we arrive but at the same measure, that of the ancients, we shall stand without that threshold; for “unless your righteousness shall exceed the righteousness of the Scribes and Pharisees, you cannot enter into the kingdom of Heaven.”
Source: Homilies on the Gospel of St. Matthew (New Advent)