14 And do not tell me, “I have accomplished the greater part of it;” but if you have not accomplished the whole, consider that you have not as yet done any thing; for this little, if neglected, is destruction to all the rest. Often indeed when men have built a house, and put on the roof, they have destroyed the whole fabric, by not making any concern of a single tile that has been shaken off from it. And one may see the same thing occur with respect to garments; for there too if a small hole is made, and not repaired, a large rent is the consequence.
And this also is frequently the case in regard to floods; for these, if they find but a small entrance, let in the whole torrent. Thou also, then, even if you have fortified yourself all around, and but a small part be left still unfortified, yet block up this also against the devil, that you may be made strong on all sides! You have seen the sickle! You have seen the head of John! You have heard the history pertaining to Saul! You have heard the manner of the Jewish captivity!
And beside all these, you have heard the sentence of Christ declaring, that not only to commit perjury, but to swear in any way, is a diabolical thing, and the whole a device of the evil one. You have heard that every where perjuries follow oaths. Putting all these things then together, write them upon your understanding. Do you not see how women and little children suspend Gospels from their necks as a powerful amulet, and carry them about in all places wherever they go. Thus do thou write the commands of the Gospel and its laws upon your mind.
Here there is no need of gold or property, or of buying a book; but of the will only, and the affections of the soul awakened, and the Gospel will be your surer guardian, carrying it as you will then do, not outside, but treasured up within; yea, in the soul's secret chambers. When you rise up then from your bed, and when you go out of your house, repeat this law: “I say unto you, Swear not at all.” And the saying will be to you a discipline; for there is no need of much labour, but only of a moderate degree of attention.
And that this is true, may thus be proved. Call your son, and frighten him, and threaten to lay a few stripes upon him, if he does not duly observe this law; and you will see, how he will immediately abstain from this custom. Is it not therefore truly absurd, that little children, out of the fear we inspire, should perform this commandment, and that we should not fear God as our sons fear us?
Source: Homilies on the Statues (New Advent)