16 Dismissing then this argument of frigid self-deception, let us not look at the case of those who fall, but at those who fashion their conduct rightly; and let us endeavour to carry along with us a memento of the present fast when it is over. And as it often happens when we have purchased a vestment, or a slave, or a precious vase, we recall again the time when we did so, and say to each other, “That slave I purchased at such a festival; that garment I bought at such a time;” so, in like manner, if we now reduce to practice this law, we shall say, I reformed the practice of swearing during that Lent; for till then I was a swearer; but from barely hearing an admonition, I have abstained from the sin.
But “the custom,” it may be objected, “is a hard thing to be reformed.” I know it is; and therefore am urgent to throw you into another custom, which is good and profitable. For when you say, it is difficult for me to abstain from what is habitual; for that very reason, I say, you should make haste to abstain, knowing for certain, that if you once make another custom for yourself of not swearing, you will want no labour afterwards. Which is the more difficult thing; not to swear, or to remain the whole day without food; and to shrivel up on water-drinking, and meagre diet? It is evident that the latter surpasses the former; yet, notwithstanding, custom has made this matter so possible and easy of execution, that when the fast comes round, although any one should exhort a thousand times, or as frequently constrain and compel one to partake of wine, or taste of any other of those things which are forbidden during fasts, yet a man would prefer to suffer anything, rather than touch the prohibited article of food; and that not for want of relish for the table, nevertheless, we bear it all with fortitude, from the habit of our conscience. And the case will be the same in regard to oaths; and just as if now, any one were to impose ever so great necessity, you would remain immovable, holding fast the habit; so also in that case, if any one should urge you ten thousand times, you would not depart from your custom.
Source: Homilies on the Statues (New Advent)