The canon has already been tampered with. The book of Enoch was considered scripture for hundreds of years...
But ir was never a part of the Tanakh (Hebrew Bible). Even before the time of Christ, the Tanakh had only 24 books, and Christ put His stamp of approval on that number by defining the OT as (1) The Law (Torah) -- 5 books, (2) The Prophets (Neviim) -- 8 books, and (3) The Psalms (Ketuvim) -- 11 books (because this section called "The Writings" begins with the book of Psalms). There was no place for Enoch in this list.
For the NT, Peter calls all of Paul's epistles Scripture. Peter calls his own epistles "a more sure word of prophecy". Luke says that his Gospel was "from above" and Paul quotes it as Scripture. Revelation plainly tells it that it was from God the Father, given to God the Son, who then had it conveyed to John by an angel. John also testifies that his testimony is true and sure (regarding his other writings). And all the other books have indications that they are also from above.