On Fasting Differences in the Church

St. Irenaeus of Lyons died ca. 202

[T]his variety in its observance has not originated in our time; but long before in that of our ancestors. It is likely that they did not hold to strict accuracy, and thus formed a custom for their posterity according to their own simplicity and peculiar mode. Yet all of these lived nonetheless in peace, and we also live in peace with one another; and the disagreement in regard to the fast confirms the agreement in the faith. (Eusebius, Ecclesiastical History Bk. 5.24.13)