To me, Gods Word. No other book have I read that is alive. The Bible lives, is God's living word. I can read a book on anything and after I have read it all the things it tells me are done, I get it. Nothing more opens up, nothing more comes to light. Its wisdom is singular, one dimensional. But God's Word! I can read over and over and yet each time have some new wisdom, to me, unfold. Some new thing for me to learn. One passage will teach me a lesson, but I read that same passage later on, a year maybe, and it opens up anew and teaches me more. Maybe even something completely different from when I first read it. The depth of wisdom the Bible contains I have never seen before. Unfathomable, infinite, alive! No book written by man is capable of this or ever has been.
The Bible is how I know Christianity is true.