ST. JOHN CHRYSOSTOM ON FASTING
I do not speak of the fast most persons keep today, 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 practice it, unless it be done according to a suitable law. 'For the wrestler is not crowned unless he strives 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 the Pharisee also fasted, but afterwards went down empty, and destitute of the fruit of fasting. The Tax collector fasted not, and yet he was accepted in preference to him who had fasted. 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, rather 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', not 'beat the air', nor while we are fighting contend with a shadow.
Fasting is a medicine, but a medicine, though it be ever so profitable, becomes frequently useless owing to the lack of skills 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, the temperament of body that admits it, the nature of the country and the season of the year, the corresponding diet, as well as various other particulars. I have said these things not to lower the value of fasting, but that we may honour fasting; for the honour of fasting consists not in abstinence from food, but in withdrawing from sinful practices; since he who limits his fasting only to an abstinence from meats, is one who lower the value of it.