Just because something seems true, doesn't make it true. Just because the Bible (for example) has had a great impact on millions of lives around the globe, and has had a positive benefit on society as a whole for centuries, it still does not mean that it is necessarily true.
The Spirit of God testifies to the believer's heart and mind that the Bible is indeed God's Word. But how about for someone who is not yet a Christian? How could a non-believer be led to assume that the Bible just may be a trustworthy guide to life? In that case we would need evidence, not just personal testimony.
Rick Warren gives us just a taste of the overwhelming amount of evidences available:
There's a world of objective evidence, both external and internal, that says the Bible is true.
External evidence proves the Bible is a historical book that you can rely on. There are 5,366 copies of the Bible dating from the time it was written to just 70 years afterwards. That fact dispels the urban legend that the Bible was changed as it passed through generations and languages.
External evidence also includes many archeological discoveries. For example, historians used to say that Solomon couldn't have had the horses the Bible says he had because no one had horses at that time. But then thousands of horse stables were found in an archeological dig.
There's also the internal evidence of the Bible itself. In a court of law, a prosecutor with two or three eyewitness accounts has a good chance of making his case. The Bible is filled with eyewitness accounts. Moses was there when the Red Sea split; Joshua was there to watch Jericho fall; the disciples saw the resurrected Jesus.
The internal evidence of the Bible also includes the fact that it tells one story with consistency - though it was written over 1,500 years on three different continents by 40 different authors from every walk of life. No human being could account for that. It's an amazing example of God's abilities.
The Bible says, "There's nothing like the written Word of God for showing you the way to salvation through faith in Christ Jesus. Every part of Scripture is God-breathed and useful one way or another—showing us truth, exposing our rebellion, correcting our mistakes, training us to live God's way. Through the Word we are put together and shaped up for the tasks God has for us." (2 Timothy 3:15-17, MSG)