5 “The Son of man,” he says, “is Lord of the Sabbath, and in that day it is by all means lawful to do good rather than do evil.” If, therefore, we do evil when we break our fast, there is no Lord's day upon which we live as we should. As to his admission that the apostles ate upon the seventh day of the week, and his remark upon this, that the time for their fasting had not then come, because of the Lord's own words, “The days will come when the Bridegroom shall be taken away from them, and then shall the children of the Bridegroom fast;” since there is “a time to rejoice, and a time to mourn,” he ought first to have observed, that our Lord was speaking there of fasting in general, but not of fasting upon the seventh day. Again, when he says that by fasting grief is signified, and that by food joy is represented, why does he not reflect what it was which God designed to signify by that which is written, “that He rested on the seventh day from all His works,”— namely, that joy, and not sorrow, was set forth in that rest? Unless, perchance, he intends to affirm that in God's resting and hallowing of the Sabbath, joy was signified to the Jews, but grief to the Christians. But God did not lay down a rule concerning fasting or eating on the seventh day of the week, either at the time of His hallowing that day because in it He rested from His works, or afterwards, when He gave precepts to the Hebrew nation concerning the observance of that day. The only thing enjoined on man there is, that he abstain from doing work himself, or requiring it from his servants. And the people of the former dispensation, accepting this rest as a shadow of things to come, obeyed the command by such abstinence from work as we now see practised by the Jews; not, as some suppose, through their being carnal, and misunderstanding what the Christians rightly understand. Nor do we understand this law better than the prophets, who, at the time when this was still binding, observed such rest on the Sabbath as the Jews believe ought to be observed to this day. Hence also it was that God commanded them to stone to death a man who had gathered sticks on the Sabbath; but we nowhere read of any one being stoned, or deemed worthy of any punishment whatever, for either fasting or eating on the Sabbath. Which of the two is more in keeping with rest, and which with toil, let our author himself decide, who has regarded joy as the portion of those who eat, and sorrow as the portion of those who fast, or at least has understood that these things were so regarded by the Lord, when, giving answer concerning fasting, He said: “Can the children of the bride-chamber mourn as long as the Bridegroom is with them?”
6. Moreover, as to his assertion, that the reason of the apostles eating on the seventh day (a thing forbidden by the tradition of the elders) was, that the time for their fasting on that day had not come; I ask, if the time had not then come for the abolition of the Jewish rest from work on that day? Did not the tradition of the elders prohibit fasting on the one hand, and enjoin rest on the other? And.yet the disciples of Christ, of whom we read that they ate on the Sabbath, did on the same day pluck the ears of grain, which was not then lawful, because forbidden by the tradition of the elders. Let him therefore consider whether it might not with more reason be said in reply to him, that the Lord desired to have these two things, the plucking of the ears of grain and the taking of food, done in the same day by His disciples, for this reason, that the former action might confute those who would prohibit all work on the seventh day, and the latter action confute those who would enjoin fasting on the seventh day; since by the former action He taught that the rest from labour was now, through the change in the dispensation, an act of superstition; and by the latter He intimated His will, that under both dispensations the matter of fasting or not was left to every man's choice. I do not say this by way of argument in support of my view, but only to show how, in answer to him, things much more forcible than what he has spoken might be advanced.
Source: Letters (New Advent)