Though the church mainly depends on the written Word of God, to some extent it relies on her unwritten traditions also. It is quite necessary since the Bible itself admits that the whole teachings and acts of Jesus are not recorded. As St. John says, "Truly Jesus did many other signs in the presence of his disciples, which are not recorded in this book" (20:30). Whatever was recorded, was written, "that you may believe that Jesus is Christ, the son of God, and that believed you may have life in His name" (Jn. 20: 31). John continues, "and there are many other things that Jesus did, which if they were written one by one, I suppose that even the world itself could not contain the books that would be written" (21 :25). Hence it is clear, there could be valid unwritten traditions from the time of Jesus. Indeed St. Paul instructs the Christians in Thessalonia, to "stand firm and hold the traditions which were taught by word or epistle" (2 The 2:15). The Thessalonians are expected to obey not only the written word (epistle) but also the spoken word.
Christ came to this world to preach the Word of God. After Jesus' death and resurrection, the same mission was handed over to his disciples, "go into all the world and preach the gospel to every creature" (Mk. 16:15). The first thirty years after Jesus' ascension, passed without significant development in the written Word of God. By this time the apostles formed communities of faithful who had deep Christ-experience. Gradually basing on the transmitted oral traditions and the living Christ-experience of each church (Lk 1 :1-2) and inspired by the Holy Spirit something began to be recorded. Thus there is a long period of oral traditions and faith experience, behind the formation of each NT book. This doesn't mean that any gospel or all the gospels taken together is a complete biography of Jesus. In any biography, everything about a person's life is never recorded. Only what is relevant is recorded, and the rest is left out. Hence one cannot assert that whatever is left out is not historical or ture. So also we cannot insist that we will accept only whatever is recorded. There could be genuine traditions which are not written in the book.
The new testament was completed almost 70 years after the origin of the Church. That doesn't imply that Church did not trust the Word of God before it is recorded. It only means that the Bible came into existence in the Church and for the Church. It was the Church which guided by the Spirit, guaranteed the authenticity of the Word of God. Hence only the Church could interpret it in proper perspective taking into account her living traditions. It is not to be left for anyone's private interpretation. St. Paul cautions against such people and says, "We are not, as so many, peddling the word of God; but as of sincerity, and as of God, we speak in the sight of God in Christ" (2 Cor. 2:17)