Reply Obj. 2: Those commandments which are given under the form of a general precept, do not bind all persons in the same way, but subject to the requirements of the end intended by the lawgiver. It will be a mortal sin to disobey a commandment through contempt of the lawgiver's authority, or to disobey it in such a way as to frustrate the end intended by him: but it is not a mortal sin if one fails to keep a commandment, when there is a reasonable motive, and especially if the lawgiver would not insist on its observance if he were present. Hence it is that not all, who do not keep the fasts of the Church, sin mortally.
Reply Obj. 3: Augustine is speaking there of those things "that are neither contained in the authorities of Holy Scripture, nor found among the ordinances of bishops in council, nor sanctioned by the custom of the universal Church." On the other hand, the fasts that are of obligation are appointed by the councils of bishops and are sanctioned by the custom of the universal Church. Nor are they opposed to the freedom of the faithful, rather are they of use in hindering the slavery of sin, which is opposed to spiritual freedom, of which it is written (Gal. 5:13): "You, brethren, have been called unto liberty; only make not liberty an occasion to the flesh." _______________________
FOURTH ARTICLE [II-II, Q. 147, Art. 4]
Whether All Are Bound to Keep the Fasts of the Church?
Objection 1: It would seem that all are bound to keep the fasts of the Church. For the commandments of the Church are binding even as the commandments of God, according to Luke 10:16, "He that heareth you heareth Me." Now all are bound to keep the commandments of God. Therefore in like manner all are bound to keep the fasts appointed by the Church.
Obj. 2: Further, children especially are seemingly not exempt from fasting, on account of their age: for it is written (Joel 2:15): "Sanctify a fast," and further on (Joel 2:16): "Gather together the little ones, and them that suck the breasts." Much more therefore are all others bound to keep the fasts.
Obj. 3: Further, spiritual things should be preferred to temporal, and necessary things to those that are not necessary. Now bodily works are directed to temporal gain; and pilgrimages, though directed to spiritual things, are not a matter of necessity. Therefore, since fasting is directed to a spiritual gain, and is made a necessary thing by the commandment of the Church, it seems that the fasts of the Church ought not to be omitted on account of a pilgrimage, or bodily works.
Obj. 4: Further, it is better to do a thing willingly than through necessity, as stated in 2 Cor. 9:7. Now the poor are wont to fast through necessity, owing to lack of food. Much more therefore ought they to fast willingly.
On the contrary, It seems that no righteous man is bound to fast. For the commandments of the Church are not binding in opposition to Christ's teaching. But our Lord said (Luke 5:34) that "the children of the bridegroom cannot fast whilst the bridegroom is with them [*Vulg.: 'Can you make the children of the bridegroom fast, whilst the bridegroom is with them?']." Now He is with all the righteous by dwelling in them in a special manner [*Cf. I, Q. 8, A. 3], wherefore our Lord said (Matt. 28:20): "Behold I am with you . . . even to the consummation of the world." Therefore the righteous are not bound by the commandment of the Church to fast.
I answer that, As stated above (I-II, Q. 90, A. 2; Q. 98, AA. 2, 6), general precepts are framed according to the requirements of the many. Wherefore in making such precepts the lawgiver considers what happens generally and for the most part, and he does not intend the precept to be binding on a person in whom for some special reason there is something incompatible with observance of the precept. Yet discretion must be brought to bear on the point. For if the reason be evident, it is lawful for a man to use his own judgment in omitting to fulfil the precept, especially if custom be in his favor, or if it be difficult for him to have recourse to superior authority. On the other hand, if the reason be doubtful, one should have recourse to the superior who has power to grant a dispensation in such cases. And this must be done in the fasts appointed by the Church, to which all are bound in general, unless there be some special obstacle to this observance.