No parts of scripture should be dismissed because of scholarly attempts to demonstrate that they are not part of the authentic sayings of Jesus or that they were later additions to the text of a particular biblical book. The authority of scripture lies, as well, in the declaration by the church of the scriptures' composition and scope. The church, by a process that has often been criticized, but never overturned, established that which we call scripture and it is that which, as a result we deal with - neither less nor more. Other books, other traditions, other understandings may incarnate the divine, but they are at best secondary to the received scriptures, and are always judged by the understandings that the received scriptures give rise to. This, coupled with the human and divine nature of what has been received, is the essence of the authority of scripture.
Authority is ascribed by human beings.
Part of the definition of what it means to be a Christian is, or ought to be, that one gives authority to the scriptures to be both the primary and the ultimate source and standard of one's faith understandings and practices. As such, the scripture is never more authoritative than when it is frequently read and frequently discussed - whether this be within a group or in private contemplation. The greater the dichotomy between what the scriptures apparently say or reveal about God and one's personal views the greater the opportunity to ascribe authority to the scriptures and allow the Spirit of God to use that ascription to produce within the hearer of the word change pleasing to God. The authority and the interpretation of the Bible go hand in hand. No look at the Bible is interpretation-free.