Christians believe things–and we live according to principles–which we could never figure out by ourselves.
Therefore we rely on some source of information that possesses infallible authority. Our faith and morals are based on the testimony of God Himself, delivered to us in writing and by word of mouth.
Now, the authority to give this testimony either resides in me myself, or it resides in someone else.
Some people actually do regard themselves as their own infallible religious authority. But it takes just a little humility and maturity to realize that being your own infallible teacher is a prescription for disaster.
Therefore my infallible teacher must be someone else.
Who is it? Could it be a politician? Joel Osteen? Oprah?
Of all the candidates for infallible teacher, the only really viable one is the Pope. The Pope can claim to hold such an office–the office of infallible teacher and shepherd established by the Son of God when He was on earth. (Matt 16:18) Jesus never promised that every Pope would be a saint. He guaranteed that there would be a spiritual fortress which the enemies of God could never conquer. Within this fortress, the true faith will always survive. The fortress is the Apostolic See of Rome, the Catholic Church.
You might say: Back off! My infallible teacher is the Bible!
1. How do you know that the Bible is the Bible (i.e. the compendium of divine teachings committed to writing)? How do you know that the Koran is NOT the Bible? Or The Origin of Species? What authority certifies that your Bible is, in fact, God’s Word?
2. If there is a dispute about what the Bible means, who has the authority to settle the question?
St Peter and his successors.