5 Thou applaudest what is said now. When you are required to swear, then remember all these things. If you quickly accomplish this law, we will advance our teaching to other and greater things. Lo! This is the second year that I am discoursing to your Charity; and I have not yet been able to explain a hundred lines of the Scriptures. And the reason is, that you need to learn of us what ye might reduce to practice at home, and of yourselves; and thus the greater part of our exhortation is consumed on ethical discourse.
But this ought not to have been so; the regulation of manners you ought to have learned at home, and of yourselves; but the sense of the Scriptures, and the speculations upon them, you might commit to us. If, however, it were necessary that you should hear such things of us, there was no need of more than one day: for what there is to be said is of no diversified or difficult character, or such as requires any elaboration. For when God declares His sentence, subtle arguments are unseasonable.
God has said, “You shall not swear.” Do not then demand of me the reasons of this. It is a royal law. He who established it, knows the reason of the law. If it had not been profitable, He would not have forbidden it. Kings bring in laws, and not all perchance profitable; for they are men, and cannot be competent to discover what is useful, like God. Nevertheless, we obey them. Whether we marry, or make wills, or are about to purchase servants, or houses, or fields, or to do any other act, we do these things not according to our own mind, but according to the laws which they ordain; and we are not entirely at liberty to dispose of the things which concern ourselves according to our own minds; but in many cases we are subject to their will; and should we do any thing that is contrary to their judgment, it becomes invalid and useless.
So then tell me, are we to pay so much respect to the laws of men, and trample under foot the law of God? What defence, or what pardon can such conduct be worthy of? He has said, “You shall not swear.” In order that you may do and speak all things with safety, do not in practice lay down a law contrary to His.
Source: Homilies on the Statues (New Advent)