6 Knowing these things then, let us avoid oaths; and let our mouth continually practise the saying, “Believe me;” and this will be to us a foundation for all pious behaviour; for the tongue, when it has been disciplined to use this one expression, is ashamed, and would blush to utter words that are disgraceful and ugly; and should it at any time be drawn away by habit, it will be checked again, by having many accusers. For when any one observes him who is not a swearer giving utterance to foul words, he will take his advantage over him, and ridicule, and exclaim tauntingly, “You who sayest in all affairs, 'Believe me,' and venturest not to utter an oath, do you disgrace your tongue with these shameful expressions?”
So that being forcibly urged by those who are with us, even if unwilling, we shall return again to a pious behaviour. “But what,” says one, “if it be necessary to take an oath?” Where there is a transgression of the law, there is no such thing as necessity. “Is it possible then,” it is replied, “not to swear at all?” What do you say? Hath God commanded, and do you dare to ask if it be possible for His law to be kept? Why, truly it is a thing impossible that His law should not be kept; and I am desirous to persuade you from present circumstances of this; that so far from its being impossible not to swear, it is impossible to swear. For behold, the inhabitants of the city were commanded to bring in a payment of gold, such as it might have seemed beyond the power of many to do; yet the greater part of the sum has been collected; and you may hear the tax gatherers saying, “Why delay, man?
Why put us off from day to day? It is not possible to avoid it. It is the law of the Emperor, which admits of no delay.” What do you say, I ask? The Emperor has commanded you to bring in your money, and it is impossible not to bring it in! God has commanded you to avoid oaths! And how do you say, it is impossible to avoid them!
Source: Homilies on the Statues (New Advent)