Because the historical science (the “earthly things”) in the Bible is true, the moral and spiritual teachings based in that history are true
From https://spot.colorado.edu/~huemer/hank.htm
Me: "Maybe your friend Karl just made the whole thing up."
John: "No way! Item 5 says, 'Hank dictated this list himself.' Besides, item 2 says, 'Use alcohol in moderation,' item 4 says, 'Eat right,' and item 8 says, 'Wash your hands after going to the bathroom.' Everyone knows those things are right, so the rest must be true, too."