8 I speak not, indeed, of such a fast as most persons keep, but of real fasting; not merely an abstinence from meats; but from sins too. For the nature of a fast is such, that it does not suffice to deliver those who practise it, unless it be done according to a suitable law. “For the wrestler,” it is said, “is not crowned unless he strive lawfully.” To the end then, that when we have gone through the labour of fasting, we forfeit not the crown of fasting, we should understand how, and after what manner, it is necessary to conduct this business; since that Pharisee also fasted, but afterwards went down empty, and destitute of the fruit of fasting.
The Publican fasted not; and yet he was accepted in preference to him who had fasted; in order that you may learn that fasting is unprofitable, except all other duties follow with it. The Ninevites fasted, and won the favour of God. The Jews, fasted too, and profited nothing, nay, they departed with blame. Since then the danger in fasting is so great to those who do not know how they ought to fast, we should learn the laws of this exercise, in order that we may not “run uncertainly,” nor “beat the air,” nor while we are fighting contend with a shadow.
Fasting is a medicine; but a medicine, though it be never so profitable, becomes frequently useless owing to the unskilfulness of him who employs it. For it is necessary to know, moreover, the time when it should be applied, and the requisite quantity of it; and the temperament of body that admits it; and the nature of the country, and the season of the year; and the corresponding diet; as well as various other particulars; any of which, if one overlooks, he will mar all the rest that have been named. Now if, when the body needs healing, such exactness is required on our part, much more ought we, when our care is about the soul, and we seek to heal the distempers of the mind, to look, and to search into every particular with the utmost accuracy.
Source: Homilies on the Statues (New Advent)