John 1:1 says: In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was in the beginning with God.
But you say Christ didn't come to start a book.
Then the very book you claim he didn't come to start, you claim as the authority by which the church was founded?
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No, the Church Jesus Christ (God) founded, is the authority, based on the authority of God Himself, which He delegated to His Church. The Bible provides evidence for that fact, as do many other sources, from that apostolic age, right down to ours, in an unbroken chain of testimony. (Of course, you need the faith Jesus spoke of to believe that, and not everybody possesses such faith.)
That Church was here long before the "Books" of the "New Testament" were even written. The proof of that, of course, is that the "Church" is mentioned in many places in those "Books" in the "New Testament", and spoken of as already existing as of the time of those writings. (Do a simple search on "Church" in any decent online searchable Bible, and see for yourself.)
(You do believe the Church would have to exist first if they were talking and writing about it as already being there in the New Testament Books -- if they were honest writers -- don't you?)
The Church (guided by God) was also the authority God chose to select exactly which "Books" belong in the Bible. God did not choose to rain down Bibles from heaven in the forms they are in today, with all those specifically selected "Books".
(Who do you think selected which "Books" belong in the Bible?)