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)