But how do we know for sure that it's god word? What if it's all written by people. What if they made it all up?
What other book ever has depth upon depth, life upon life.
"For the word of God is alive and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart." (Hebrews 4:12).
Those of us who are saved, who are baptized into the holy Spirit, have the whole bible written in our hearts. The same life that is in the bible (the power and truth of God) is in the believer (that is the Holy Spirit is in the believer in the power and truth of God) and testifies to the life in the bible so we know from God's Spirit in us that the bible is God's life in truth. The words when we read them come alive matching scripture upon scripture, yet we can match one scripture with a bunch of scriptures one day and take the same one scripture and match it with a bunch of scriptures another day and on and on and there is never contradictions.
"This is the covenant I will make with them after that time, says the Lord. I will put my laws in their hearts, and I will write them on their minds (Hebrews 10:16).
4But when the kindness of God our Savior and His love for mankind appeared,
5He saved us, not on the basis of deeds which we have done in righteousness, but according to His mercy, by the washing of regeneration and renewing by the Holy Spirit,
6whom He poured out upon us richly through Jesus Christ our Savior (Titus 3:5). A saved person is washed of their sins and the Holy Spirit keeps renewing life and truth in them since the Holy Spirit is fire that purifies and water that cleanses.
He who believes in Me, as the Scripture said, 'From his innermost being will flow rivers of living water.'"
39But this He spoke of the Spirit (John 7:39).
John the baptist speaking of Jesus said, "After me comes one who is more powerful than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire" (Matthew 3:11).
True salvation isn't just believing from knowledge because it sounds good, true salvation is believing from real faith because the truth of who Jesus is (God in the flesh) who died to pay our sin debts and raise us to life as new creations is real spiritual truth to the person and they know it as truth. The same is with the bible. We know the bible as real spiritual truth. This truth is not dogmatism of belief. Rather it is spiritual life from God.