The authority of apostolic tradition in early Christianity cannot be overstated.
It can, however, be misconstrued, and that is done every day.
When Christians hear about tradition in the context of Christianity, we usually immediately think of Roman Catholicism. We assume that the authority of tradition means that some church with a large organization—usually the Roman Catholic Church—gets to determine doctrine for all Christians.
Nothing could be further from the truth.
The key word in apostolic tradition is not tradition, but apostolic.
Notice the emphasis here. The idea behind tradition, to Irenaeus, was that it helped determine what the apostles taught. He even adds that this is only necessary where we don't have the apostles' writings:
Apostolic tradition was whatever the apostles taught.
The early Christians wanted it whether by letter or by word of mouth (2 Thess. 2:15), and the apostles' churches were a terrific link to what the apostles taught.
It's not just Irenaeus who says this. It is all over the early Christian writings:
More quotes on this subject can be found on the Quotes About Apostolic Tradition page.
It seems almost certain to me that the writings that make up our New Testament were chosen for their association with the apostles. Even Hebrews, which is believed by very few modern scholars to be written by Paul, was thought by the early church to be Paul's. (There's a great page on Origen's thoughts on Hebrews that help give a picture of the early Christian view that "the men of old handed it down as Paul's.")
Writings like The Epistle of Barnabas and 1 Clement were in some collections of Scripture also because of their association to apostles. Many thought The Epistle of Barnabas was written by the Barnabas of Acts, and there's a Clement mentioned as a "fellow-worker" of Paul's in Philippians 4:3.
In the same way, all four Gospels were considered apostolic in early Christianity. Matthew and John were apostolic for obvious reasons. In early Christianity, however, Mark was considered to be Peter's Gospel because Mark traveled with Peter. The same was true for the Gospel of Luke. Luke was Paul's companion; so, his Gospel carried Paul's authority.
Tertullian (c. A.D. 200) explained it this way, calling the apostles' companions "apostolic men":
As you can see, in early Christianity truth was intimately tied to apostolic teaching. Even the Bible was put together to preserve "the faith once for all delivered to the saints" (Jude 3)
Even the Roman Catholic Church admits that tradition should not carry any authority unless it originated with the apostles. The Catholic Encyclopedia states:
The problem is that the RCC claims an authority to teach that allows them to claim that their traditions are apostolic even when there is abundant evidence to the contrary. The lateness of traditions like the sinlessness and assumption of Mary (the teaching that here body was caught up to heaven after death), and the egregiously anti-scriptural nature of traditions like indulgences make it clear that the RCC is inventing their own traditions without even any real effort to solely preserve apostolic tradition. Nonetheless, statements like the one above make it clear that they still—officially if not practically—hold the doctrine that all authoritative tradition must have originated with the apostles.
None of the traditions here presented as authoritative—papal infallibility, the assumption of Mary, her sinlessness and perpetual virginity, and the blasphemous assertion that she is "Queen over all things"—can be traced even to the pre-Nicene church (before A.D. 325), much less back to the apostles.
This sort of tradition, based on the wishes of organizations raised up long after apostolic times, are the antithesis of true apostolic tradition. The apostolic tradition of the early churches was meant to prevent the invention of such falsehoods.
In the early churches, they were not limited to the Bible. Apostolic truth could be found, as we saw earlier, by resorting to apostolic churches.
Irenaeus, quoted earlier, had it even better. An old man when he wrote his Against Heresies in the 180's, he had listened to Polycarp, the revered bishop of Smyrna, in his younger years. Polycarp had been appointed to his position by the apostle John, and Irenaeus describes what it was like to hear him:
Irenaeus understood that the reason this was wonderful is because Polycarp was thus a witness to the apostolic tradition, that body of truth, "the faith once for all delivered to the saints," that he himself—for Irenaeus was a bishop, too—was to preserve unchanged for future generations:
The problem is, we have neither Polycarp nor apostolic churches today.
True, churches like the Roman Catholic Church in Rome or the Greek Orthodox Church in Antioch could claim to have some sort of physical succession from apostolic times, but after 2,000 years that is no witness at all to the apostolic traditions, especially when we consider what has happened over those 2,000 years.
Since that is true, do we need a testimony to apostolic tradition, or are the Scriptures enough?
Since the writings of the early Church could be considered a testimony to the traditions that the apostles handed down, it would be possible to argue that these ought to carry authority for us today. It is not just the early Church that honored apostolic tradition, but Paul said to do so as well (1 Cor. 11:2; 2 Thess. 2:15).
However, I would rather dodge the question.
Why should it matter whether we must consult the early church witness to apostolic tradition when we can consult them?
Look around you. Are the Scriptures, by themselves, leading Christians to walk in the ways of the apostles?
Do we not argue and divide? Jesus said a house divided against itself cannot stand, and Paul pleaded with his churches to be of one mind (Matt. 12:25; 1 Cor. 1:10-13; Phil. 1:27 - 2:4). It is apparent that the Scriptures alone are not solving our differences of opinion.
All who have come before us have humbly consulted those who came before them in order to know better how to please God and how not to damage the church of Jesus Christ, which is both his body and his bride. John Wesley, for example, wrote:
It is not a matter of requirement, but of need, that we should familiarize ourself with those who can, through their writings, still bear witness to us of the traditions of the apostles.
We cannot speak to Polycarp, as Irenaeus did, but Irenaeus tells us what we can do:
Irenaeus' statement is still true because Polycarp's letter to the Philippians is indeed still available.
Numerous other testimonies to the faith once for all delivered to the saints are also available. First Clement is a 1st century document written from the church at Rome to the church at Corinth. The anonymous Letter to Diognetus may be just as early and claims to be written by a disciple of the apostles. The Letter of Barnabas, though it certainly does not have a Scriptural quality to it, was honored in the 2nd century church, as was the later Shepherd of Hermas.
The testimonies of the apostolic faith in the apostles' churches are many. The collection of those writing is far larger than the New Testament, are replete with references to the Scripture, and the faith contained in them is always recounted "in harmony with the Scriptures."
Why would we, who care with the early churches what the apostles taught, not do all we can to find out what the apostolic tradition is?