Sacred Tradition helps us to understand Sacred Scripture. The successors to the Apostles understood that.
Pope Clement I
"Then the reverence of the law is chanted, and the grace of the prophets is known, and the faith of the Gospels is established, and the Tradition of the Apostles is preserved, and the grace of the Church exults" (Letter to the Corinthians 11 [A.D. 80]).
"Papias [A.D. 120], who is now mentioned by us, affirms that he received the sayings of the Apostles from those who accompanied them, and he moreover asserts that he heard in person Aristion and the presbyter John. Accordingly he mentions them frequently by name, and in his writings gives their Traditions [concerning Jesus]. . . . [There are] other passages of his in which he relates some miraculous deeds, stating that he acquired the knowledge of them from Tradition" (Fragment in Eusebius, Church History 3:39 [A.D. 312]).
Eusebius of Caesarea
"At that time [A.D. 150] there flourished in the Church Hegesippus, whom we know from what has gone before, and Dionysius, bishop of Corinth, and another bishop, Pinytus of Crete, and besides these, Philip, and Apolinarius, and Melito, and Musanus, and Modestus, and finally, Irenaeus. From them has come down to us in writing, the sound and orthodox faith received from Tradition" (Church History 4:21).
Irenaeus of Lyons
"As I said before, the Church, having received this preaching and this faith, although she is disseminated throughout the whole world, yet guarded it, as if she occupied but one house. She likewise believes these things just as if she had but one soul and one and the same heart; and harmoniously she proclaims them and teaches them and hands them down, as if she possessed but one mouth. For, while the languages of the world are diverse, nevertheless, the authority of the Tradition is one and the same" (Against Heresies 1:10:2 [A.D. 189]).
“That is why it is surely necessary to avoid them [heretics], while cherishing with the utmost diligence the things pertaining to the Church, and to lay hold of the Tradition of truth. . . . What if the Apostles had not in fact left writings to us? Would it not be necessary to follow the order of Tradition, which was handed down to those to whom they entrusted the Churches?” (ibid., 3:4:1).
"It is possible, then, for everyone in every church, who may wish to know the truth, to contemplate the Tradition of the Apostles which has been made known throughout the whole world. And we are in a position to enumerate those who were instituted bishops by the Apostles and their successors to our own times–men who neither knew nor taught anything like these heretics rave about.
"But since it would be too long to enumerate in such a volume as this the successions of all the churches, we shall confound all those who, in whatever manner, whether through self-satisfaction or vainglory, or through blindness and wicked opinion, assemble other than where it is proper, by pointing out here the successions of the bishops of the greatest and most ancient church known to all, founded and organized at Rome by the two most glorious Apostles, Peter and Paul, that church which has the Tradition and the faith which comes down to us after having been announced to men by the Apostles.
“With this church, because of its superior origin, all churches must agree–that is, all the faithful in the whole world–and it is in her that the faithful everywhere have maintained the Apostolic Tradition” (ibid., 3:3:1-2).
Eusebius of Caesarea
"A question of no small importance arose at that time [A.D. 190]. For the parishes of all Asia . . . held that the fourteenth day of the moon, on which day the Jews were commanded to sacrifice the lamb, should be observed as the feast of the Savior’s Passover. . . . But it was not the custom of the churches in the rest of the world to end it at this time, as they observed the practice which, from Apostolic Tradition, has prevailed to the present time, of terminating the fast [of Lent] on no other day than on that of the resurrection of our Savior [Sunday]" (Church History 4:23).