According to Matthew Henry in his commentary on 2 Timothy 3:16 and surrounding, the Bible itself speaks to its wondrous origins. Henry points to:
-the majesty of its style
-the truth, purity and sublimity of the doctrines contained in it
-from the harmony of its several parts
-from its power and efficacy on the minds of multitudes that converse with it
-from the accomplishment of many prophecies
-from the uncontrollable miracles
Thank you God for revealing yourself and your truth to me through the Bible.