13 The licentiousness of the tongue is a great snare, and needs a strong bridle. Therefore also some one says. “His own lips are a powerful snare to a man, and he is snared by the words of his own mouth.” Above all the other members, then, let us control this; let us bridle it; and let us expel from the mouth railings, and contumelies, and foul and slanderous language, and the evil habit of oaths. For again our discourse has brought us to the same exhortation. But I had arranged with your charity, yesterday, that I would say no more concerning this precept, forasmuch as enough has been said upon it on all the foregoing days.
But what is to become of me? I cannot bear to desist from this counsel, until I see that you have put it in practice; since Paul also, when he says to the Galatians, “Henceforth let no man trouble me,” appears again to have met and addressed them. Such are the paternal bowels; although they say they will depart, yet they depart not, until they see that their sons are chastened. Have ye heard today what the prophet speaks to us concerning oaths; “I lifted up my eyes, and I saw,” says he, “and, behold, a flying sickle, the length thereof twenty cubits, and the breadth thereof ten cubits; and he said to me, What do you see?
And I said, I see a flying sickle, twenty cubits in length, and ten cubits in breadth. It shall also enter into the house,” says he, “of every one that swears in my name, and shall remain in the midst, and shall pull down the stones and the wood.” What, forsooth, is this which is here spoken? And for what reason is it in the form of a “sickle,” and that a “flying sickle,” that vengeance is seen to pursue the swearers? In order that you may see that the judgment is inevitable, and the punishment not to be eluded.
For from a flying sword some one might perchance be able to escape, but from a sickle, falling upon the neck, and acting in the place of a cord, no one can escape. And when wings too are added, what further hope is there of safety? But on what account does it pull down the stones and the wood of the swearer's house? In order that the ruin may be a correction to all. For since it is necessary that the earth must hide the swearer when dead; the very sight of his ruined house, now become a heap, will be an admonition to all who pass by and observe it, not to venture on the like, lest they suffer the like; and it will be a lasting witness against the sin of the departed.
The sword is not so piercing as the nature of an oath! The sabre is not so destructive as the stroke of an oath! The swearer, although he seems to live, is already dead, and has received the fatal blow. And as the man who has received the halter, before he has gone out of the city and come to the pit, and seen the executioner standing over him, is dead from the time he passed the doors of the hall of justice: so also the swearer.
Source: Homilies on the Statues (New Advent)