When Jesus ascended into heaven, He did NOT yell down: "READ MY BOOK!"
Our Lord did NOT leave us a book; He left us TWELVE MEN, with Peter as the Rock, the Shepherd of the flock of Jesus' foundling Church. Thus we have the Apostolic Tradition and the Magesterium (all the bishops and the Bishop of Rome--the pope).
Peter's successors have been around in an unbroken line since then. Their names are engraved in granite in the Bascilica of St. Peter. The four names NOT there are the four popes who chose to serve out their papacy in Avignon. They were duly and correctly elected but did not serve in Rome.
Unbroken line of Peter's successors...what else to expect from what Jesus founded?
Another good question is to ask what the BIBLE tells us is the foundation of all truth for the Christian?
;-))