33 Again you have heard that it was said to them of old, Thou shalt not forswear thyself: but thou shalt perform thy oaths to the Lord. 34 But I say to you not to swear at all.
Gloss: It is written in Leviticus, “Thou shalt not forswear thyself in my name” [Lev 19:12]. . . . That they should not make gods of the creature, they are commanded to render to God their oaths, and not to swear by any creature. . . . As it is written in Deuteronomy, “Thou shalt fear the Lord thy God, and shalt swear by his name” [Deut 6:13].
Jerome: 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.
The Saviour does not here forbid to swear by God, but by the Heaven, the Earth, by Jerusalem, by a man’s head. . . . He who swears, shew 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 [Deut 6:13].
Augustine: 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” [Gal 1:20]. So the Romans, “God is my witness, whom I serve in my spirit” [Rom 1:9].
The Apostle has used oaths in his Epistles, and by this shews us how that ought to be taken, “I say unto you, Swear not at all,” namely, lest by allowing ourselves to swear at all we come to readiness in swearing, from readiness we come to a habit of swearing, and from a habit of swearing we fall into perjury. And so the Apostle is not found to have used an oath but only in writing, the greater thought and caution which that requires not allowing of slip of the tongue.
In his writings, as writing allows of greater circumspection, the Apostle is found to have used an oath in several places, that none might suppose that there is any direct sin in swearing what is true; but only that our weak hearts are better preserved from perjury by abstaining from all swearing whatever.
Yet is the Lord’s command so universal, “Swear not at all,” that He would seem to have forbidden it even in writing. But since it would be an impiety to accuse Paul of having violated this precept, especially in his Epistles, we must understand the word “at all” as implying that, as far as lays in your power, you should not make a practice of swearing, not aim at it as a good thing in which you should take delight.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. I (London: Rivington, 1842).