Catena Aurea on Matthew 5:33–37: Augustine of Hippo

But what we could not understand by mere words, from the conduct of the saints we may gather in what sense should be understood what might easily be drawn the contrary way, unless explained by example. 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.

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.