12 Knowing these things, then, brethren, and gathering up what has been now advanced, as well as what has been said before; let us at last desist from this evil custom, yea, I pray and beseech you all! For if in the old dispensation, when the Jews had not the strictest moral wisdom required of them, but much condescension was extended to them, such wrath was the effect of one oath; such capture and captivity; what punishment is it likely that those who swear should now be subjected to, after an express law forbidding the practice, and so large an addition of precepts.
Is it, indeed, all that is required, that we come to the assembly, and hear what is spoken? Why truly it is a reason for greater condemnation, and for more inevitable punishment, that we are continually hearing, and yet do not what is bidden! What excuse shall we have, or what pardon, if assembling here from earliest youth to latest old age, and enjoying the advantage of so much instruction, we remain just like them, and do not take pains to correct a single defect. Let no one henceforth allege custom.
For this is the very thing at which I am indignant and provoked, that we are not able to get the better of custom. And, pray, if we do not get the better of custom, how can we get the better of concupiscence, which has its root even in the principles of our nature; for it is natural to feel desire; but to desire wickedly, comes after of choice. But this practice of swearing takes not even its first principle from nature, but from mere negligence.
Source: Homilies on the Statues (New Advent)