The truth is found nowhere except in the catholic church, the sole depository of apostolic doctrine. The apostles like a rich man depositing money in a bank, have lodged in their hands most copiously all things pertaining to truth: so that every man, whosoever will, can draw from her water of life, for, she is the entrance of life, and all others are robbers. Among us we have recourse to the most ancient churches which the apostles held constant intercourse, and from them we learn what is certain and clear in regard to present questions. (Against Heresies, Iranaeus, ca 150 AD)
The church was plagued with error from its very start. However, in God’s kindness we have recorded the words of a man who was the student of Poloycarp, who was the student of John the apostle. He writes this about 60 years after John death, and explains to his readers that the treatment of heresies belongs in the hands of the one apostolic, ancient church with whom the apostles had regular communication and discipleship occurring. This is astounding. Would we commend the same thing? Well, it depends upon who one asks. Perhaps this question really gets us to ask another: “what is the role of apostolic teaching other than what is found in scripture?” It is obvious that there existed such a thing; the fact is that most churches did not have the final canon of the New Testament yet, they only had pieces, and they also had books that were not canonical along with them. The canon was not finally agreed upon until the 4th century. So what was this other knowledge? It was the liturgical practice and piety that was handed down by the apostles, a practice which was practiced day by day in the life of the church at the time of this writing. It was songs and hymns, prayers and liturgies, creeds and ritual acts, and acts of piety, many of which are recorded in the scriptures. In summary, it was the life of the early church that was spread throughout the world by word and deed, and eventually canonized in Holy Scripture. Apostolic tradition and the apostolic New Testament are not two different things; they come from the same fountain, contain the same truth and work towards the same end.
So then, brothers, stand firm and hold to the traditions that you were taught by us, either by our spoken word or by our letter. 2 Thess 2: 15