First came the Old Testament and Sacred Tradition. Then came the Church. Then came the New Testament. After Jesus founded His Church there only existed the Old Testament and Sacred Tradition. The New Testament came from what was known by word of mouth. The Holy Spirit guided the Church in this way. Nowhere will you find in the New Testament anything about the Holy Spirit limiting Himself to the written word. In fact, nowhere will you find Jesus saying anything about writing anything down. This is certainly not to deny the importance of the New Testament. It is the Holy Word of God. But just as God spoke to the early Church directly through the Holy Spirit, He still does.
So we recognize the power of the Holy Spirit in the authority the Jesus gave to St. Peter and his successors. There are teachings in the Church that go back to the time before the compiling of the New Testament that are not in it. For example the identity of the Blessed Virginís parents, Anna and Joachim.
For more on Sacred Tradition, see:
Fr. Vincent Serpa, O.P.