The Church had the authority to interpret the Scripture, since she was the only authentic depository of Apostolic kerygma. This kerygma was unfailingly kept alive in the Church, as she was endowed with the Spirit.
The Church was still teaching viva voce, commending and furthering the Word of God. And viva vox Evangelii [the living voice of the Gospel] was indeed not just a recitation of the words of the Scripture. It was a proclamation of the Word of God, as it was heard and preserved in the Church, by the ever abiding power of the quickening Spirit.