The short answer is: because God said so. But such quick answers don’t always satisfy our human curiosity. So let’s look at some facts:

There are more extant ancient copies of the Scriptures than there are of any other ancient work.

Today’s copies of the Bible are in 99.6% exact agreement with the ancient copies -- try getting that kind of message fidelity in a simple game of “Telephone”! And that 0.4% errant material involves grammar and spelling errors, not matters of faith.

The Bible is really a library of books, written in many literary styles, through the cooperation of God with dozens of human authors, across geographical distances, written over the span of many hundreds of years. No other literary effort even comes close.

The Bible has been a best-seller from the start, and it continues to outsell all other books (even Harry Potter) by orders-of-magnitude.

The Bible has survived (and continues to survive) targeted attacks from Islam, Nazism, Communism, etc. (For a thrilling book about one man’s efforts to sneak Bibles behind the Iron Curtain during the Cold War, read God’s Smuggler.)

Logically speaking, these facts shouldn’t be. If of purely human origin, the Bible would have long ago been corrupted or destroyed -- or both. Clearly, Someone (with a capital S) is protecting it.

All that being said, does this mean every single word is literally true? No -- the Catholic Church does not teach a literal reading of the Bible. God/Jesus didn’t intend for it to be. But importantly, the Bible is true in everything we need for our salvation. For example, when Jesus fed the thousands, did He start with 5 loaves and 2 fish, or 2 loaves and 5 fish? Does that matter? No, what matters is that He alone can satisfy our true longings -- for salvation, for love, and for Heaven. And His miraculous feeding of so many is a sign of His divine power to take care of us. So the Bible is true, just not always literal.