How do we know that the Bible is not the sole rule of faith?
The Bible does not claim to be the sole rule of faith. Paul wrote, “What you have heard from me before many witnesses entrust to faithful men who will be able to teach others also” (2 Tim. 2:2). And he instructed, “Hold to the traditions which you were taught by us, either by word of mouth or by letter” (2 Thess. 2:15).
These oral teachings and traditions have been handed down and entrusted to the Church, and they remain as much a part of the full Christian faith as the Bible. To ignore them is no less a tragedy than to ignore the Bible.