25 Forsooth continence is a virtue, not of the body, but of the soul. But the virtues of the soul are sometimes shown in work, sometimes lie hidden in habit, as the virtue of martyrdom shone forth and appeared by enduring sufferings; but how many are there of the same virtue of mind, unto whom trial is wanting, whereby what is within, in the sight of God, may go forth also into the sight of men, and not to men begin to exist, but only become known? For there was already in Job patience, which God knew, and to which He bore witness: but it became known unto men by test of trial: and what lay hid within was not produced, but shown, by the things that were brought on him from without.
Timothy also certainly had the virtue of abstaining from wine, which Paul took not from him, by advising him to use a moderate portion of wine, “for the sake of his stomach and his often infirmities,” otherwise he taught him a deadly lesson, that for the sake of the health of the body there should be a loss of virtue in the soul: but because what he advised could take place with safety to that virtue, the profit of drinking was so left free to the body, as that the habit of continence continued in the soul.
For it is the habit itself, whereby any thing is done, when there is need; but when it is not done, it can be done, only there is no need. This habit, in the matter of that continence which is from sexual intercourse, they have not, unto whom it is said, “If they contain not, let them be married.” But this they have, unto whom it is said, “Whoso can receive, let him receive.” Thus have perfect souls used earthly goods, that are necessary for something else, through this habit of continence, so as, by it, not to be bound by them, and so as by it, to have power also not to use them, in case there were no need.
Nor does any use them well, save who has power also not to use them. Many indeed with more ease practise abstinence, so as not to use, than practise temperance, so as to use well. But no one can wisely use them, save who can also continently not use them. From this habit Paul also said, “I know both to abound, and to suffer want.” Forsooth to suffer want is the part of any men soever; but to know to suffer want is the part of great men. So, also, to abound, who cannot? But to know also to abound, is not, save of those, whom abundance corrupts not.
Source: On the Good of Marriage (New Advent)