5 “Again, you have heard that it was said to them of old time, You shall not forswear yourself, but shall perform unto the Lord your oaths. But I say unto you, swear not at all.”
Why did He go straightway not to theft, but to false witness, passing over that commandment? Because he that steals, does upon occasion swear also; but he that knows not either swearing or speaking falsehood, much less will he choose to steal. So that by this He has overthrown the other sin likewise: since falsehood comes of stealing.
But what means, “You shall perform unto the Lord your oaths?” It is this, “you shall be true in swearing.” “But I say unto you, swear not at all.”
Next, to lead them farther away from swearing by God, He says, “Neither by Heaven, for it is God's throne, nor by the earth, for it is the footstool of His feet; nor by Jerusalem, for it is the city of the great King:” still speaking out of the prophetical writings, and signifying Himself not to be opposed to the ancients. This was because they had a custom of swearing by these objects, and he intimates this custom near the end of his Gospel.
But mark, I pray you, on what ground He magnifies the elements; not from their own nature, but from God's relation to them, such as it had been in condescension declared. For because the tyranny of idolatry was great, that the elements might not be thought worthy of honor for their own sake, He has assigned this cause, which we have mentioned, which again would pass on to the glory of God. For He neither said, “because Heaven is beautiful and great,” nor, “because earth is profitable;” but “because the one is God's throne, the other His footstool;” on every side urging them on towards their Lord.
“Neither by your head,” says He, “because you can not make one hair white or black.”
Here again, not as wondering at man, has He withdrawn him from swearing by his head (for so man himself would be worshipped), but as referring the glory to God, and signifying that you are not master even of yourself, and of course therefore not of the oaths made by your head. For if no one would give up his own child to another, much more will not God give up His own work to you. For though it be your head, yet is it the property of another; and so far from being master thereof, you shall not be able to do with it, no not the least thing of all. For He said not, “You can not make one hair grow;” but, “Not so much as change its quality.”
“But what,” it may be said, “if any one should require an oath, and apply constraint?” Let the fear of God be more powerful than the constraint: since, if you are to bring forward such excuses, you will keep none of the things which are enjoined.
Yea, for first with respect to your wife you will say, “what if she be contentious and extravagant;” and then as to the right eye, “what if I love it, and am quite on fire?” and of the unchaste look, “what then, if I cannot help seeing?” and of our anger against a brother, “what if I be hasty, and not able to govern my tongue?” and in general, all His sayings you may on this wise trample under foot. Yet surely with regard to human laws you dare not in any case use this allegation, nor say, “what then if this or that be the case,” but, willing or unwilling, you receive what is written.
And besides, you will never have compulsion to undergo at all. For he that has hearkened unto those former blessings, and has framed himself to be such as Christ enjoined, will have no such constraint to endure from any, being held in reverence and veneration by all.
“But let your yea, be yea; and your nay, nay: for that which exceeds these comes of the evil one.”
What is it then that “exceeds yea” and “nay”? It is the oath, not the perjury. For this latter is quite acknowledged, and no man needs to learn that it is of the evil one; and it is not an excess, but an opposite: whereas an excess means something more, and added over and above: which kind of thing swearing is.
“What then,” says one, “was it of the evil one? And if it was of the evil one, how was it a law?” Well, this same thing you will say concerning the wife also; how is that now accounted adultery, which was before permitted?
What now may one reply to this? That the precepts then uttered had reference to the weakness of them who were receiving the laws; since also to be worshipped with the vapor of sacrifice is very unworthy of God, just as to lisp is unworthy of a philosopher. That kind of thing accordingly was now laid down to be adultery, and swearing to be of the evil one, now that the principles of virtue have advanced. But if these things had been, from the first, laws of the devil, they would not have attained to so great goodness. Yea, for had those not been forerunners in the first place, these which we now have would not have been so easily received. Do not thou then require their excellency now, when their use is past: but then, when the time was calling for them. Or rather, if you will, even now: yea, for now also is their virtue shown: and most of all for the very cause, by reason of which we find fault with them. For their appearing such now, is the greatest commendation of them. For had they not brought us up well, and made us meet for the reception of the greater precepts, they would not have appeared such.
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?
Source: Homilies on the Gospel of St. Matthew (New Advent)