That’s just the point...the “authority” is what God has decreed as the truth. It isn’t based on anyone’s opinion but God’s word was established AS His authority because “holy men of God spoke as they were moved by the Holy Spirit” (2 Peter 1:21).
The truths we believe are based on what God has revealed IS the truth, not what some self-proclaimed prophet declares. It is why Christians have always believed in the basic tenets of the faith taught by Jesus Christ and His Apostles and which they passed down to faithful men and women to this day. The written Scriptures are preserved to be the guide for what is the truth and is not dependent on oral traditions, storytelling or word of mouth. No one says the gospel is only available to literate people, far from it because faith comes by hearing and hearing by the word of God. The Scriptures are the user’s manual, so to speak. It is there so we can have a more sure way of knowing truth from error.
Your believing or rejecting of a truth doesn’t change whether or not it IS the truth. It is not relative. Everything I believe by faith can be proved by Scripture. I don’t have to be a prophet to trust what God has spoken and to rest in the faith that His word confirms.
Truths? No. Beliefs, yes.