The Lenten Fast is frequently referred to by writers of primitive days as an established and well known custom, which had been sanctioned by Apostolical authority.

The Lenten Fast is however frequently referred to by writers of primitive days as an established and well known custom, which had been sanctioned by Apostolical authority. The probability is, that even from the first—from the time in which "the Bridegroom was taken away''—His followers thus in sorrow kept the anniversary of His Passion, although the duration of this season, and the rules by which its observance was regulated, may not have been definitely settled until the age immediately succeeding that of the Apostles.
Philo, who was contemporary with the early disciples, and is even said "to have had familiar conversation with Peter at Rome, whilst he was proclaiming the Gospel to the inhabitants of that city, (Eusebius' Eccles. Hist., liber 11., chap. 17, p. 66.)'' refers to this season in his description of the Christians of Alexandria, who were converted by St. Mark. "This author,"—says Eusebius, in his history composed about A. D 321—"has accurately described and stated in his writings, the exercises performed by them," (i.e. the Christians of Alexandria in the days of St. Mark), "which are still in vogue among us at the present day, and especially at the festival of our Saviour's passion, which we are accustomed to pass in fasting and watching, and in the study of the divine word. These are the same customs that are observed by us alone at the present day, particularly the vigils of the Great Festival, (Eusebius' Eccles. Hist. lib. ii, chap. 17 p. 68.) meaning by this the Passion week, called by the Greek Fathers the Great Week.

It is also mentioned incidentally by Irenaeus, who lived but ninety years after the death of St. John, and was trained up under the martyr Polycarp, who had himself been a disciple of that last surviving Apostle. When alluding to a difference of opinion with regard to the time in which it should be kept, he shows that the custom itself was ancient, even in his day. His words are: "This diversity existing among those that observe it, is not a matter that has just sprung up in our time, but long ago, among those before us.'' (Ibid, lib. v. chap. 24, p. 210.)

Tertullian too, who lived within one hundred years of the Apostle St. John's departure, has unwittingly as it were, recorded his testimony to the general belief of the Church in the Apostolical Authority of this season. Having erred from the faith, and embraced the heresy of the Montanists, he found the voice of the Church against him, when he endeavoured to introduce the new fasts which Montanus had commanded. Thus therefore he argues against her authority, in defence of his party. "They" (i.e. the Catholic Christians) "accuse us that we observe fasts of our own, peculiar to ourselves. They object therefore unto us novelty, and prescribe against the unlawfulness of that, saying, it is either to be judged Heresy, if presuming as men, we so dogmatize, or we are to be pronounced false prophets, if we inculcate these fasts, as from the Spirit]; whilst on either hand we hear them denounce an anathema against us. For as to what pertains to fast, they argue, that there are certain days constituted by God They surely think, that in the Gospel those days are determined for facts, in which the Bridegroom was taken away, and that those days only are now the legitimate days of Christian fasts, all legal and prophetical old observances being antiquated or abolished. Therefore as to other fasting, it is to be indifferent, according to every man's occasions and causes, at his own judgment, not of command." (That is, as Montanus inculcated the necessity of his fast, by pretended command from God.) "-And that thus the Apostles observed the rule of fasting, imposing no other yoke of certain or set fasts to be kept of all in common. And ye prescribe against us, that the solemn times for this matter, are to he believed already constituted in the Scriptures, or in the tradition of our Elders, and that no further observance is to be super added, for the unlawfulness of innovation." (Tertullian De Jejuniis, chap. 1, 2, 13.)

The history, object, and proper observance of the holy season of Lent by Kip, William Ingraham, 1811-1893