Wherefore we must reply that, as Augustine states (De Mendacio xv), "when the Apostle employs an oath in his epistles, he shows how we are to understand the saying, 'I say to you, not to swear at all'; lest, to wit, swearing lead us to swear easily and from swearing easily, we contract the habit, and, from swearing habitually, we fall into perjury.
— from Summa Theologica, Part II-II (Secunda Secundae) Translated by Fathers of the English Dominican Province by Thomas, Aquinas, Saint