In the first place, as regards that law about swearing: accomplish that; establish it firmly: and let him that has kept it make known him that has not, and call him to account withal and rebuke him sternly. For the (supra, Hom. viii.) appointed time (ἡ προθεσμία), is at hand and I am holding inquisition in the matter, and him that is found guilty I will banish and exclude. But God forbid that any such should be found among us; rather may it appear, that all have strictly kept this spiritual watchword. And as in war it is by the watchword that friends and strangers are shown, so let it be now; for indeed now also we are engaged in a war; that we may know our brethren that are properly such. For what a good thing it is that we should have this to be our cognizance both here and in a foreign land! What a weapon this, against the very head of the devil! A mouth that cannot swear will soon both engage God in prayers, and smite the devil a deadly blow. A mouth that cannot swear will also be incapable of using insulting language. Cast out this fire from your tongue, as you would from a house: this fire, drag it out. Give your tongue a little rest: make the sore less virulent. Yea, I beseech you, do this, that I may go on to set you another lesson: for as long as this is not rightly done, I dare not pass on to any other. Let this lesson be got perfectly, and you shall have a consciousness of the achievement, and then I will introduce you to other laws, or rather not I, but Christ. Implant in your soul this good thing, and little by little you shall be a paradise of God, far better than that paradise of old. No serpent among you, no deadly tree, nor any such thing. Fix this habit deep. If this be done, not ye only that are present shall be benefitted, but all that are in all the world; and not they alone, but those that are to succeed hereafter. For a good habit having once entered, and being kept by all, will be handed on to long ages, and no circumstances shall be able to erase it. If he that gathered sticks on the sabbath was stoned—the man that is doing a far more heinous work than that gathering, the man that is amassing a load of sins, for such is the multitude of oaths, what shall he undergo? What shall he not have to endure? You will receive great assistance from God, if this be well achieved by you. If I were to say, Be not abusive, immediately you will plead to me your indignation; should I say, Be not envious, you will urge some other excuse. But in this case you have nothing of the kind to say. On which account I began with the easy precepts, which indeed is also the uniform practice in all arts. And thus one comes to the higher duties, by learning first those which are easier far. How easy it is you will see, when by the grace of God having succeeded in this, you shall receive another precept.
Put it in my power to speak out boldly, in the presence both of Gentiles and of Jews, and, above all, of God. Yea, I entreat you by the love, by the pangs wherewith I have travailed for your birth, “my little children.” I will not add what follows, “of whom I travail in birth again;” nor will I say, “until Christ be formed in you.”. For I am persuaded, that Christ has been formed in you. Other language I will use towards you; “My brethren, dearly beloved and longed for, my joy and my crown.” Believe me that I shall use no other language. If at this moment there were placed upon my head ten thousand richly-jewelled royal crowns, they could not give me the joy which I feel at your growth in holiness; or rather, I do not think the monarch himself has such a joy, as that wherewith I joy over you. Let him have come home, victorious over all the nations at war with him, let him have won many other crowns besides the crown of his right; and receive other diadems as tokens of his victory: I do not think he would joy over his trophies, as I joy over your soul's progress. For I exult, as if I had a thousand crowns on my head; and well may I rejoice. For if by the grace of God you achieve this good habit, you will have gained a thousand battles far more difficult than his; by wrestling and fighting with malicious demons, and fiendish spirits, with the tongue, not with sword, but by the will. For consider how much is gained, if so be that you do succeed! You have eradicated, first, a heinous habit; secondly, an evil conceit, the source of all evil, namely, the opinion that the thing is indifferent and can do no hurt; thirdly, wrath; fourthly, covetousness; for all these are the offspring of swearing. Nay, hence you will acquire a sure footing in the way to all other virtues. For as when children learn their letters, they learn not them alone, but by means of them are gradually taught to read; so shall it be with you. That evil conceit will no longer deceive you, you will not say, This is indifferent; you will no longer speak by mere habit, but will manfully stand against all, so that having perfected in all parts that virtue which is after God, you may reap eternal blessings, through the grace and loving-kindness of His Only-Begotten Son, 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)