It would be true, in a sense, to say that there is but one source of revelation, namely, divine Tradition, understanding thereby the body of revealed truth handed down.
St. Basil clearly shows the need for a teaching authority to interpret Scripture properly. Sacred Tradition and the Magisterium work together to prevent distortions of Scripture and the Gospel.
The only certain guide as to inspiration and canonicity of all the books of Sacred Scripture is the authoritative pronouncement of the Church. Moreover, since the Church is the divinely appointed custodian of revelation, it is evidently her office to preserve not merely the letter of Scripture, but also their meaning, The Church, therefore, is the authentic and infallible interpreter of Scripture
These, then, are the two sources of divine revelation: Tradition preserved by the living and infallible teaching authority of the Church, and Scripture, the inspired word of God.
But the task of authentically interpreting the word of God, whether written or handed on, has been entrusted exclusively to the living teaching office of the Church, whose authority is exercised in the name of Jesus Christ. This teaching office is not above the word of God, but serves it, teaching only what has been handed on, listening to it devoutly, guarding it scrupulously and explaining it faithfully in accord with a divine commission and with the help of the Holy Spirit, it draws from this one deposit of faith everything which it presents for belief as divinely revealed
peace