Apostolic Tradition plays a vital part in God's revelation to us. We know this since the Word of the New Testament was entrusted to the apostles by Jesus Christ, and He consecrated them in truth as He consecrated Himself: "Dedicate them to yourself by means of the truth, your word is truth. I sent them into the world, just as you sent me into the world. And for their sake I dedicate myself to you, in order that they, too, may be truly dedicated to you". (John 17: 17 - 19)
He also prayed for us, His followers down the centuries who would believe in Him: "I pray not only for them, but also for those that believe in me because of their message. I pray that they may all be one. Father! May they be in us, just as you are in me and I am in you. May they be one, so that the world will believe that you sent me". (John 17: 20 - 21)
So, when the Church was born on the day of Pentecost, and received the power to proclaim the Gospel to the ends of the earth, there was no written New Testament. Yet, the message of the Gospel was alive and burning in the hearts of the apostles, who had been Jesus' closest companions and eyewitnesses to His ministry.
The apostles were eager to communicate this message to others, to show them the path to salvation, so they handed on the apostolic tradition in two ways:
Oral Tradition: It was handed on by the spoken word they preached, the example of the lives they lived, and the institutions they established. The content of what they handed on was what they had received, whether in the words Jesus spoke, or from His works and the way of life, or by the prompting of the Holy Spirit.
Written Tradition: The message of salvation was committed to writing by the apostles and others associated with them, under the inspiration of the Holy Spirit. This fact is established by Paul exhorting the early Christians to "...Stand firm and hold on to those truths that we taught you, both in our preaching and in our letter". (2 Thess 2: 15)
But soon, false teachings made their appearance: first there were false and fanciful stories being circulated as the gospel; and second there were wrong interpretations that differed from what the apostles taught.
It became, therefore, the primary responsibility of the successors of the apostles to hand on and safe guard the genuine apostolic tradition about the life and teachings of Jesus. All this prompted Paul to request Timothy to stay in Ephesus because, "some people there are teaching false doctrines and you must order them to stop". (1 Tim 1: 3)
Paul further cautioned Titus to appoint church leaders who, "must hold firmly to the message which can be trusted and which agrees with the doctrine. In this way he will be able to encourage others with the true teaching and also to show the error of those who are opposed to it". (Titus 1: 9)
From about 25 years after Pentecost till the end of the 1st Century, the teachings of the apostles were committed to writing, and became available to the Christians in the churches being set up in Asia, Europe and North Africa.
At the same time, many spurious "gospel narratives" and "letters" also began to circulate. The early Christians, always eager to receive some "teaching" about the Lord, were in danger of being exposed to these false teachings.
It was the responsibility of the bishops to discern which writings faithfully conveyed the teaching of the apostles, and which did not. So, for almost four centuries the Church grew and spread without a canonized New Testament, but with only the Oral and Written Traditions discerned by the bishops under the guidance of the Holy Spirit.
A series of Councils of Bishops was held towards the end of the 4th Century, to discern which books were truly inspired by the Holy Spirit. In the process several writings were rejected. Finally, the Council of Carthage compiled a "Canon", or a definitive list of inspired books, which received the final approval. This is the New Testament as we have it today, and it is the one that every Christian acknowledges to be the inspired Word of God and swears by.
Yet, even when there was agreement on the inspired books that comprised the New Testament, there were frequent disagreements on the correct interpretation, the genuine meaning of some texts. It is to the credit of the early church that it understood, from the promise of Jesus in Scripture, that the Holy Spirit was truly guiding the bishops, and especially the meetings of the bishops (Councils, Synods) which were called for the specific purpose of discussing these difficult matters.
These Councils followed in the tradition of the Council of Jerusalem recorded in Scripture (Acts 15), and the formal statements and documents of these Councils are a part of the Church's Sacred Tradition.
Many central documents of Christianity, firmly believed by all Christians, were in fact formulated during the various Councils held in the first few centuries of the Church's existence. As these serve the important purpose of clarifying the true meaning of Scripture, it is unthinkable to reject them and insist on "scripture alone" without an authoritative interpretation.