The Nevi'im (Prophets) and Ketuvim (the "Writings" or Hagiographa) were canonized by 'Anshei HaKenesset HaGedolah (the "Men of the Great Assembly"). To accept them is to accept the authority of the canonizing body.
The "new testament" was canonized by the Catholic/Orthodox churches in the "fourth century." To accept them is also to accept the authority of the canonizing bodies. Needless to day, the Jewish People do not accept their authority any more than they accept the authority of Mohammed or Joseph Smith to make up new religions.
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