25 As to the succeeding paragraphs with which he concludes his treatise, they are, like some other things in it which I have not thought worthy of notice, even more irrelevant to a discussion of the question whether we should fast or eat on the seventh day of the week. But I leave it to yourself, especially if you have found any help from what I have already said, to observe and dispose of these. Having now to the best of my ability, and as I think sufficiently, replied to the reasonings of this author, if I be asked what is my own opinion in this matter, I answer, after carefully pondering the question, that in the Gospels and Epistles, and the entire collection of books for our instruction called the New Testament, I see that fasting is enjoined. But I do not discover any rule definitely laid down by the Lord or by the apostles as to days on which we ought or ought not to fast. And by this I am persuaded that exemption from fasting on the seventh day is more suitable, not indeed to obtain, but to foreshadow, that eternal rest in which the true Sabbath is realized, and which is obtained only by faith, and by that righteousness whereby the daughter of the King is all glorious within.
26. In this question, however, of fasting or not fasting on the seventh day, nothing appears to me more safe and conducive to peace than the apostle's rule: “Let not him that eats despise him that eats not, and let not him which eats not judge him that eats:” “for neither if we eat are we the better, neither if we eat not are we the worse;” our fellowship with those among whom we live, and along with whom we live in God, being preserved undisturbed by these things. For as it is true that, in the words of the apostles, “it is evil for that man who eats with offense,” it is equally true that it is evil for that man who fasts with offense. Let us not therefore be like those who, seeing John the Baptist neither eating nor drinking, said, “He has a devil;” but let us equally avoid imitating those who said, when they saw Christ eating and drinking, “Behold a man gluttonous, and a drunkard, a friend of publicans and sinners.” After mentioning these sayings, the Lord subjoined a most important truth in the words, “But Wisdom is justified of her children;” and if you ask who these are, read what is written, “The sons of Wisdom are the congregation of the righteous:” they are they who, when they eat, do not despise others who do not eat; and when they eat not, do not judge those who eat, but who do despise and judge those who, with offense, either eat or abstain from eating.
Source: Letters (New Advent)