This is an extremely important question. “Because the Bible says it’s God’s Word” is the short answer, but one that most people won’t be satisfied with. It’s certainly true that the Bible says it is God’s Word (2 Peter 1:20, 21). Just think of how many prophetic books in the Bible begin with, “The word of the LORD came to” or how often the prophets introduce their words with, “This is what the LORD says.”
On the other hand, most people who pose the question want corroboration from an independent, outside source that proves that the Bible is God’s Word. And that, of course, is the problem. What independent outside sources are there? God is one, of course—but the Bible is the only Word of God that we have, and the Bible itself tells us not to expect any other (Isaiah 8:20, Revelation 22:18, 19). God isn’t going to speak from heaven and tell us, “This is my book. Believe it!”
Human beings are the only other possibility. But human beings are hardly unbiased or impartial. As they are by nature, they have every reason not to believe the Bible’s claims about itself. That’s one reason why arguments that believers find persuasive—like biblical prophecies that were clearly and obviously fulfilled, or the fact that the Bible is still around despite centuries of being vehemently attacked and suppressed—don’t necessarily convince unbelievers.
Fortunately, the Bible doesn’t need independent corroboration, because it is self-authenticating. The best advice we can give to someone who is wondering whether the Bible is God’s Word is, “Read it, and you’ll find out.” You’ll discover a book by people you’ve never met, and yet who know you better than you know yourself. And you’ll find them leading you straight to your loving Savior. That is finally the only “proof” that the Bible really needs.