Hebrews 6:16

"For men verily swear by the greater, and an Oath for confirmation is to them an end of all strife. But God because He could swear by no greater, sware by Himself." Well, who then is He that sware unto Abraham? Is it not the Son? No, one says. Certainly indeed it was He: however, I shall not dispute thereon. So when He the Son sweareth the same oath, "Verily, verily, I say unto you," is it not plain that it was because He could not swear by any greater? For as the Father sware, so also the Son sweareth by Himself, saying, "Verily, verily, I say unto you." He here reminds them also of the oaths of Christ, which He was constantly uttering. "Verily, verily, I say unto thee, he that believeth on Me shall never die."

What is, "And an oath for confirmation is to them an end of all strife"? It is instead of, "by this every doubtful question is solved": not this, or this, but every one.

"an end of all dispute." And the conclusion of all contradiction and dispute to confirm what is said is the oath. For indeed many things are spoken and disputed on each side; but the oath, coming last and confirming, resolves all doubts.

That is, an oath resolves the dispute in every contradiction. For much is said and objected in argument from both sides, but the oath, being the final word and confirming, resolves everything that is doubtful.

A promissory oath is made for preserving fidelity, and this is done in three ways, namely for restraining malice, for settling controversy, and for clearing away infamy. First, it is made for restraining malice, so that a man may neither deny the true nor affirm the false; and this is the reason for the usefulness of the oath. Second, it is made for settling controversy. Whence the Apostle says to the Hebrews: "The end of all controversy is an oath." Third, it is made for clearing away infamy.

317. – Then when he says, Men indeed swear by a greater than themselves, he states the reason for the foregoing. In regard to this he does three things: first, he mentions a human custom; secondly, the reason behind the custom; thirdly, the fruit of the promise (v. 18).

318. – Two human customs are mentioned: one in regard to that by which one swears; the other in regard to the effect of an oath: in all their disputes an oath is final for confirmation. Now that by which one swears is greater, and reasonably so. For swearing is nothing more than settling a doubt. Therefore, just as in the sciences nothing is settled except by something more certain, so, because nothing is more certain with men than God, one swears by Him, as by something greater and more certain.

319. – But sometimes one swears by the Son, Who is less; as when one says, 'by Christ' or by the creature, as Joseph swore by the health of Pharaoh (Gen. 42:16). I answer that there are two ways of swearing by God: one is by a simple attestation, as when one says, 'By God, it is so.' As if to say: God is my witness that it is as I say: 'For God is my witness, who I serve in spirit' (Rom. 1:9). Another way is by execration, which occurs when something, such as the head of the soul or something of that sort, is offered unto God for vengeance, if it is not so. This is the way the Apostle swore in 2 Cor (1:23): 'But I call God to witness upon my soul.' As if to say: I offer my soul as witness, for which I take God's name. And this is very solemn. But one swears by a creature not as such, but inasmuch as some sign of God's power is resplendent in it. For since all power is from God (Rom. 13:1), then, inasmuch as someone exercises power over a group, if he swears by it, he swears by God, Whose power is reflected in it. This is the way Joseph swore by the health of Pharaoh. This, therefore, is what he says, Men swear by one greater than themselves.

320. – But it should be noted that those who are otherwise known as perjurers must be kept from taking oaths, because the greatest reverence should be shown an oath, and from their past actions it is presumed that they will not show an oath proper reverence. Likewise, children should not be compelled to take an oath before the years of puberty, because they do not yet have perfect use of reason to know what reverence must be paid an oath. Also persons of high stations; because an oath is required of those about whose words or actions there is doubt. But it detracts from persons of high station, when one doubts the truth of what they say. Hence, in the Decretals (q. 2, ch. 4) it says that a priest should not take an oath for some slight cause. But the causes in which it is lawful to swear are the following: for strengthening peace, as Laban swore (Gen. 31:48); secondly, for preserving one's good name; thirdly, for pledging fealty, as vassals swore to their lord; fourthly, for having obedience fulfilled, if something honorable is commanded by a superior; fifthly, for obtaining security; sixthly, for attesting to a truth, which is the way the Apostle swore: 'God is my witness, whom I serve in my spirit' (Rom. 1:9).

321. – Then when he says, and an oath is final for confirmation, he mentions the effect of an oath, which is to put an end to all controversy. For just as in the sciences one stops when he has reduced his proof to the first principles, so is the divine law, when one comes to the first truth, he stops and this is the time, when the first truth is invoked as a witness: 'The master of the house shall be brought to the gods and he shall swear' (Ex. 22:8). Thus all questions and all controversies are settled.