7 “How shall we,” says our author, “escape sharing the condemnation of the Pharisee, if we fast twice in the week?” As if the Pharisee had been condemned for fasting twice in the week, and not for proudly vaunting himself above the publican. He might as well say that those also are condemned with that Pharisee, who give a tenth of all their possessions to the poor, for he boasted of this among his other works; whereas I would that it were done by many Christians, instead of a very small number, as we find. Or let him say, that whosoever is not an unjust man, or adulterer, or extortioner, must be condemned with that Pharisee, because he boasted that he was none of these; but the man who could think thus is, beyond question, beside himself. Moreover, if these things which the Pharisee mentioned as found in him, being admitted by all to be good in themselves, are not to be retained with the haughty boastfulness which was manifest in him, but are to be retained with the lowly piety which was not in him; by the same rule, to fast twice in the week is in a man such as the Pharisee unprofitable, but is in one who has humility and faith a religious service. Moreover, after all, the Scripture does not say that the Pharisee was condemned, but only that the publican was “justified rather than the other.”
8. Again, when our author insists upon interpreting, in connection with this matter, the words of the Lord, “Unless your righteousness shall exceed the righteousness of the scribes and Pharisees, you shall not enter into the kingdom of heaven,” and thinks that we cannot fulfil this precept unless we fast oftener than twice in the week, let him mark well that there are seven days in the week. If, then, from these any one subtract two, not fasting on the seventh day nor on the Lord's day, there remain five days in which he may surpass the Pharisee, who fasts but twice in the week. For I think that if any man fast three times in the week, he already surpasses the Pharisee who fasted but twice. And if a fast is observed four times, or even so often as five times, passing over only the seventh day and the Lord's day without fasting—a practice observed by many through their whole lifetime, especially by those who are settled in monasteries—by this not the Pharisee alone is surpassed in the labour of fasting, but that Christian also whose custom is to fast on the fourth, and sixth, and seventh days, as the Roman community does to a large extent. And yet your nameless metropolitan disputant calls such an one carnal, even though for five successive days of the week, excepting the seventh and the Lord's day, he so fast as to withhold all refection from the body; as if, forsooth, food and drink on other days had nothing to do with the flesh, and condemns him as making a god of his belly, as if it was only the seventh day's repast which entered into the belly.
Source: Letters (New Advent)