The Bible’s authority comes from its being the inspired witness to the revelations of God. God did not dictate the Bible but communicated His will through His prophets and apostles.
Religion is humankind’s attempt to reach God and Christianity is God’s attempt to reach humankind. Generally God reveals Himself in His creation. He gives His Special Revelation through specific revelations and disclosures. He revealed His name, His covenant, His Commandments etc. The Bible is God’s infallible special revelation and it is reliable and trustworthy in His revelation of Himself and His will.
The canon we have today comes from thousands of years of critical testing. In some cases, where the authorship is clear, as in the case of The Gospels, their inspiration is accredited due to their closeness to the source, Jesus. Other books are included in the canon because spiritual leaders who we know and trust in history proclaimed them as inspired. The external test lies in the number of copies of portions, in some cases, and, whole books in others, though separated by centuries are found to be substantively and functionally identical. With over five thousand manuscripts in hand – some dating back to 400 years before Christ – we find less that a tenth or a percent difference in the final collection.
This compares to the minuscule number of late copies of the writings of Plato, Aristotle, or Shakespeare. As archeological discoveries reveal time and again the truth of the Biblical record there is now more reason to believe God’s special revelation today than on the day of Pentecost immediately following the resurrection of Jesus in which 3000 accepted God’s promise through Jesus Christ.