Well, then, let us speak to you today from the Old Testament. What does it tell us? “Accustom not your mouth to swearing; neither use yourself to the naming of the Holy One.” And why? “For as a servant that is continually beaten shall not be without a blue mark, so he that swears.” See the discernment of this wise man. He did not say, “Accustom not to swearing” your mind, but “your mouth”; because being altogether an affair of the mouth, thus it is easily remedied. For at last it becomes a habit without intention; as for instance, there are many who entering the public baths, as soon as they have passed the threshold, cross themselves (σφραγίζονται). This the hand has got to do, without any one's bidding, by force of habit.
Again, at the lighting of a candle, often when the mind is intent on something else, the hand makes the sign. In the same way also the mouth, without concurrence of the mind, articulates the word, from mere habit, and the whole affair is in the tongue. “Neither use yourself,” he says, “to the naming of the Holy One. For as a servant that is continually beaten shall not be without a blue mark, so he that swears.” He speaks not here of false oaths, but he cuts down all oaths, and to them also assigns their punishment.
Why then, swearing is a sin. For such in truth is the soul; full of all these ulcers, all these scars. But you do not see them? Yes, this is the mischief of it; and yet you might see if you wished; for God has given you eyes. With eyes of this kind did the Prophet see, when he said, “My wounds stink, and are corrupt, because of my foolishness.” We have despised God, we have hated that good Name, we have trodden Christ under foot, we have lost all reverence, none names the Name of God with honor.
Yet if you love any one, even at his name you start to your feet; but God you thus continually invoke, and make nothing of it. Call upon Him for the benefit of your enemy; call upon Him for the salvation of your own soul; then he will be present, then you will delight Him; whereas now you provoke Him to anger. Call upon Him as Stephen did; “Lord,” he said, “lay not this sin to their charge.” Call upon Him as did the wife of Elkanah, with tears and sobs, and prayers. I prevent you not, rather I earnestly exhort you to it.
Call upon him as Moses called upon Him, yea, cried, interceding for those who had driven him into banishment. For you to make mention at random of any person of consideration, is taken as an insult: and do you bandy God about in your talk, in season, out of season? I do not want to hinder you from keeping God always in your mind: nay, this I even desire and pray for, only that you should do this, so as to honor Him. Great good would this have done us, if we had called upon God only when we ought, and for what we ought.
And why, I would ask, were such miracles wrought in the Apostles' times, and not in ours? And yet it is the same God, the same Name. But no, the case is not the same. For then they called upon Him only for those objects which I have mentioned; whereas we call upon Him not for these, but quite other purposes.— If a man refuse to believe you, and that is why you swear, say to him, “Believe me:” however, if you will needs make oath, swear by yourself. I say this, not to set up a law against Christ's law; God forbid; for it is said.
Let your yea be yea, and your nay, nay: but by way of coming down to your present level, that I may more easily lead you to the practice of this commandment, and divert you from this tyrannical habit. How many who have done well in other respects, have been undone by these practices! Shall I tell you why it was permitted the ancients to take oaths? (for to take false oaths, was not permitted to them either.) Because they swore by idols. But are you not ashamed to rest in laws, by which they in their infirmity were led on to something better?
It is true, when I take a Gentile in hand, I do not immediately lay this injunction upon him, but in the first place I exhort him to know Christ; but if the believer, who has both learned Him and heard Him, must needs crave the same forbearance with the Gentile, what is the use, what the gain (of his Christianity?)— But the habit is strong, and you cannot detach yourself from it? Well then, since the tyranny of habit is so great, transfer it into another channel. And how is this to be done?
You will ask. What I have said often, I say also now; let there be many monitors (λογισταὶ), let there be many examiners and censors (ἐ ξετασταὶ, δοκιμασταί). Say, if you chance to put on your mantle inside out, you allow your servant to correct your mistake, and are ashamed to learn of him, although there is much to be ashamed of in this; and here when you are getting hurt to your soul, are you ashamed to be taught better by another? You suffer your menial to put your dress in order, and to fasten your shoes, and will you not endure him that would put your soul in order?
Let even your menial, your child, your wife, your friend, your kinsman, your neighbor, be your teachers on this point. For as when a wild beast is hunted down from all sides, it is impossible for it to escape; so he that has so many to watch him, so many to reprove him, who is liable to be struck at from all sides, cannot help being on his guard. The first day he will find it hard to put up with, and the second, and the third; but after that it will come easier, and, the fourth passed, there will not even be anything to do.
Make the experiment, if you doubt me; take it into consideration, I beseech you. It is not a trifling matter to be wrong in, nor yet to come right in; on both sides it is great for evil and for good. May the good be effected, through the grace and loving-mercy of our Lord Jesus Christ, to Whom with the Father and the Holy Ghost be glory, power, and honor, now and ever, world without end. Amen.
Source: Homilies on Acts (New Advent)