When it comes to answering the above question, I believe there are three kinds of people to consider: 1.) The true follower of Christ has an absolute assurance that the Bible is the Word of God because he has studied it and has applied its principles to life situations. The question for him has been settled in the affirmative; 2.) The seeker has a true desire to find the truth and is willing to honestly investigate the claims of the Bible for himself; 3.) The critic will look at the facts through distorted eyes that only allow him to see what he wants to see. When there is a desire to discredit the Bible and/or God nothing will stand in his way. It is not to him that I am writing.
In Part One, we will offer no proof for the origin of Scripture, but we will first look at what the Bible says about itself.The Bible is just as relevant as it was 3,500 years ago and never makes an attempt to defend itself. There is no need to. We do, however read of the promise of preservation in Psalm 12:6, 7--"The words of the LORD are pure words: as silver tried in a furnace of earth, purified seven times. Thou shalt keep them, O LORD, thou shalt preserve them from this generation for ever."
Even Christians today question the validity of the Bible. Like Jehudi of old, they take the parts they don't like and throw them into the fire (Jeremiah 36:23), but II Timothy 3:16 gives a different thought. All Scripture is given by inspiration. That is, Scripture is God-breathed--from the very breath of God. Every last jot and tittle comes from God Himself and is given for our benefit. From a practical viewpoint, we are to live by every Word that comes from the mouth of God (Matthew 4:4).
Notice I Peter 1:20, 21. "Knowing this first, that no prophecy of the scripture is of any private interpretation. For the prophecy came not in old time by the will of man:but holy men of God spake as they were moved by the Holy Ghost."
First thing--Scripture does not have a private interpretation. Everyone can know it and understand it through the teaching of the Holy Spirit and it has the same doctrinal meaning to everyone. God is saying the same thing to Lutherans, Mormons, Baptists, Catholics, and anyone else who reads the Bible. It makes me wonder why we have so many different denominations. God's Word is the same for all without any of the denominational spins.
Secondly, realize that holy men spoke and recorded God's message as it was given to them by the Holy Ghost. It was not their philosophy, not their beliefs, but God's Word transmitted to them. For the Christian, this should be easy enough to understand. For the true seeker, we will look at some other proofs in Part Two.