51 “Again,” says He, “you have heard that it has been said to them of old time, You shall not forswear yourself, but shall perform unto the Lord your oath: But I say unto you, Swear not at all; neither by heaven, for it is God's throne; nor by the earth, for it is His footstool; neither by Jerusalem, for it is the city of the great King. Neither shall you swear by your head, because you can not make one hair white or black. But let your communication be Yea, yea; Nay, nay: for whatsoever is more than these comes of evil.” The righteousness of the Pharisees is not to forswear oneself; and this is confirmed by Him who gives the command not to swear, so far as relates to the righteousness of the kingdom of heaven. For just as he who does not speak at all cannot speak falsely, so he who does not swear at all cannot swear falsely. But yet, since he who takes God to witness swears, this section must be carefully considered, lest the apostle should seem to have acted contrary to the Lord's precept, who often swore in this way, when he says, “Now the things which I write unto you, behold, before God I lie not;” and again, “The God and Father of our Lord Jesus Christ, which is blessed for evermore, knows that I lie not.” Of like nature also is that asseveration, “For God is my witness, whom I serve with my spirit in the gospel of His Son, that without ceasing I make mention of you always in my prayers.” Unless, perchance, one were to say that it is to be reckoned swearing only when something is spoken of by which one swears; so that he has not used an oath, because he has not said, by God; but has said, “God is witness.” It is ridiculous to think so; yet because of the contentious, or those very slow of apprehension, lest any one should think there is a difference, let him know that the apostle has used an oath in this way also, saying, “By your rejoicing, I die daily.” And let no one think that this is so expressed as if it were said, Your rejoicing makes me die daily; just as it is said, By his teaching he became learned, i.e. by his teaching it came about that he was perfectly instructed: the Greek copies decide the matter, where we find it written, Νὴ τὴν καύχησιν ὑμετέραν, an expression which is used only by one taking an oath. Thus, then, it is understood that the Lord gave the command not to swear in this sense, lest any one should eagerly seek after an oath as a good thing, and by the constant use of oaths sink down through force of habit into perjury. And therefore let him who understands that swearing is to be reckoned not among things that are good, but among things that are necessary, refrain as far as he can from indulging in it, unless by necessity, when he sees men slow to believe what it is useful for them to believe, except they be assured by an oath. To this, accordingly, reference is made when it is said, “Let your speech be, Yea, yea; Nay, nay;” this is good, and what is to be desired. “For whatsoever is more than these comes of evil;” i.e., if you are compelled to swear, know that it comes of a necessity arising from the infirmity of those whom you are trying to persuade of something; which infirmity is certainly an evil, from which we daily pray to be delivered, when we say, “Deliver us from evil.” Hence He has not said, Whatsoever is more than these is evil; for you are not doing what is evil when you make a good use of an oath, which, although not in itself good, is yet necessary in order to persuade another that you are trying to move him for some useful end; but it “comes of evil” on his part by whose infirmity you are compelled to swear. But no one learns, unless he has had experience, how difficult it is both to get rid of a habit of swearing, and never to do rashly what necessity sometimes compels him to do.
52. But it may be asked why, when it was said, “But I say unto you, Swear not at all,” it was added, “neither by heaven, for it is God's throne,” etc., up to “neither by your head.” I suppose it was for this reason, that the Jews did not think they were bound by the oath, if they had sworn by such things: and since they had heard it said, “You shall perform unto the Lord your oath,” they did not think an oath brought them under obligation to the Lord, if they swore by heaven, or earth, or by Jerusalem, or by their head; and this happened not from the fault of Him who gave the command, but because they did not rightly understand it. Hence the Lord teaches that there is nothing so worthless among the creatures of God, as that any one should think that he may swear falsely by it; since created things, from the highest down to the lowest, beginning with the throne of God and going down to a white or black hair, are ruled by divine providence. “Neither by heaven,” says He, “for it is God's throne; nor by the earth, for it is His footstool:” i.e., when you swear by heaven or the earth, do not imagine that your oath does not bring you under obligation to the Lord; for you are convicted of swearing by Him who has heaven for His throne, and the earth for His footstool. “Neither by Jerusalem, for it is the city of the great King;” a better expression than if He had said, “My [city];” although, however, we understand Him to have meant this. And, because He is undoubtedly the Lord, the man who swears by Jerusalem is bound by his oath to the Lord. “Neither shall you swear by your head.” Now, what could any one suppose to belong more to himself than his own head? But how is it ours, when we have not the power of making one hair white or black? Hence, whoever should wish to swear even by his own head, is bound by his oath to God, who in an ineffable way keeps all things in His power, and is everywhere present. And here also all other things are understood, which could not of course be enumerated; just as that saying of the apostle we have mentioned, “By your rejoicing, I die daily.” And to show that he was bound by this oath to the Lord, he has added, “which I have in Christ Jesus.”
53. But yet (I make the remark for the sake of the carnal) we must not think that heaven is called God's throne, and the earth His footstool, because God has members placed in heaven and in earth, in some such way as we have when we sit down; but that seat means judgment. And since, in this organic whole of the universe, heaven has the greatest appearance, and earth the least—as if the divine power were more present where the beauty excels, but still were regulating the least degree of it in the most distant and in the lowest regions—He is said to sit in heaven, and to tread upon the earth. But spiritually the expression heaven means holy souls, and earth sinful ones: and since the spiritual man judges all things, yet he himself is judged of no man, he is suitably spoken of as the seat of God; but the sinner to whom it is said, “Earth you are, and unto earth shall you return,” because, in accordance with that justice which assigns what is suitable to men's deserts, he is placed among things that are lowest, and he who would not remain in the law is punished under the law, is suitably taken as His footstool.
Source: Our Lord's Sermon on the Mount (New Advent)