The root of the word 'authority' goes back to the word 'author'. The authority of Scripture resides in its ultimate author: God. The Bible is to be believed and submitted to because it is the word of God.
As “witness” the Bible is not a closed self-referential universe but rather a vehicle pointing beyond itself to 'what it is really all about', i.e. the text's true subject matter. The witnessed-to reality can be variously labelled as Scripture's “substance” (res), “Word”, “God”, “Christ”, “the divine reality which has entered time and space”, “Gods one plan of redemption” or “Gospel”. Whatever nomenclature this ultimate subject of the Bible receives, it is the life and sustenance of the church.