Matthew 5:37

But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these comes from evil.
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Augustine of Hippo

AD 430
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.

Augustine of Hippo

AD 430
Cont. Faust., xix. 23: Inasmuch as the sin of perjury is a grievous sin, he must be further removed from it who uses no oath, than he who is ready to swear on every occasion, and the Lord would rather that we should not swear and keep close to the truth, than that swearing we should come near to perjury. Serm. in Mont., i, 17: This precept also confirms the righteousness of the Pharisees, not to forswear; inasmuch as he who swears not at all cannot forswear himself. But as to call God to witness is to swear, does not the Apostle break this commandment when he says several times to the Galatians, “The things which I write unto you, behold, before God, I lie not.” That this does not mean, your boasting has caused my dying daily, but is an oath, is clear from the Greek, which is . de Mendac. 15: But what we could not understand by mere words, from the conductof the saints we may gather in what sense should be understood what might easily be drawn the contrary way, unless explained by exam...

Cornelius a Lapide

AD 1637
But let your communication be, &c.—i.e, a simple affirmation, or negation. For what is more than these, Gr. περισσὸν. The Syriac has, what is added beyond these. In the Hebrew Gospel ascribed to S. Matthew , we have אין אין ain, ain, כן כן ken, ken—that is no, no, Song of Solomon , so. In this passage a simple affirmation or negation is opposed to an oath; so in S. James (v12); and it means that whatever is added to these in the way of swearing, is of evil. So S. Chrysostom and S. Jerome, or rather Paulinus, Epist. ad Celantium. Of evil. Evil here may be taken either in the masculine or the neuter gender. If the masculine the devil is meant, who, as a ringleader of all iniquity, incites thee to swear without necessity, and so draws thee on by degrees to swear falsely, which is the sin of perjury. So Theophylact, Maldonatus, and others. If you take the neuter, it means cometh of vice, either your own or another"s—that is to say, the custom of swearing arises either ...

Glossa Ordinaria

AD 1480
Non occ.: The Lord has hitherto taught to abstain from injuring our neighbour, forbidding anger with murder, lust with adultery, and the putting away a wife with a bill of divorce. He now proceeds to teach to abstain from injury to God, forbidding not only perjury as an evil in itself, but even all oaths as the cause of evil, saying, “Ye have heard it said by them of old, Thou shalt not forswear thyself.”
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Hilary of Poitiers

AD 368
Otherwise; They who live in the simplicity of the faith have not need to swear, with them ever, what is is, what is not is not; by this their life and their conversation are ever preserved in truth.
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Jerome

AD 420
This was allowed under the Law, as to children; as they offered sacrifice to God, that they might not do it to idols, so they were permitted to swear by God; not that the thing was right, but that it were better done to God than to daemons. Lastly, consider that the Saviour does not here forbid to swear by God, but bythe Heaven, the Earth, by Jerusalem, by a man’s head. For this evil practice of swearing by the elements the Jews had always, and are thereof often accused in the prophetic writings. For he who swears, show either reverence or love for that by which he swears. Thus when the Jews swore by the Angels, by the city of Jerusalem, by the temple and the elements, they paid to the creature the honour and worship belonging to God; for it is commanded in the Law that we should not swear but by the Lord our God. Therefore Evangelic verity does not admit an oath, since the whole discourse of the faithful is instead of an oath.

John Chrysostom

AD 407
He has prohibited anyone from swearing by his head, for in doing so one would be worshiping himself. Rather, Jesus intends to refer all glory to God, signifiying that human beings are not finally masters of themselves. Oaths made by the head are thus discredited. For if no one would relinquish his own child to another, how much more will God refuse to relinquish his own work to you? For though it be your head, yet it remains the property of another. For he did not say, “You cannot make one hair grow” but “You cannot make one hair white or black” or change its quality. What is it then that exceeds a simple yes or no? It is the oath, not the perjury. For lying is openly acknowledged to be wrong, and no one needs to learn that it is of the Adversary. It is not an excess but a deficiency. An excess, though, means something more, something over and above the statement itself. This is the nature of an oath. Someone might then object: If the evil one is the source of all oaths, how could they...

John Chrysostom

AD 407
For no man can swear often, but he must sometimes forswear himself; as he who has a custom of much speaking will sometimes speak foolishly. Note how He exalts the elements of the world, not from their own nature, but from the respect which they have to God, so that there is opened no occasion of idolatry. Or; “of evil,” that is, from their weakness to whom the Law permitted the use of an oath. Not that by this the old Law is signified to be from the Devil, but He leads us from the old imperfection to the new abundance.

John Chrysostom

AD 407
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 d...
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Rabanus Maurus

AD 856
Having forbidden swearing, He instructs us how we ought to speak, “Let your speech be yea, yea; nay, nay.” That is, to affirm any thing it is sufficient tosay, ‘It is so;’ to deny, to say, ‘It is not so.’Or, “yea, yea; nay, nay,” are therefore twice repeated, that what you affirm with the mouth you should prove in deed, and what you deny in word, you should not establish by your conduct.
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Theophylact of Ochrid

AD 1107
Lest you ask, "How then will people believe what I say?" He says, "They will believe you if you always tell the truth and never swear at all." For no one is disbelieved more than he who is eager to swear that he speaks the truth. The Lord says that swearing, which is more than "Yea" and "Nay," is of the devil. But, you will ask, is the law of Moses, which bids us to swear, also evil? Learn, then, that at that time it was not evil to swear. But after Christ, it is evil. And so it is with circumcision and, in short, with all the Judaic practices. So also, to nurse at the breast is proper for infants, but shameful for adults.

Knowing this first, that no prophecy of the scripture is of any private interpretation - 2 Peter 1:20

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