Rather than a not-so-subtle deflection of the thread topic into a drive-by shot at Martin Luther, why don’t you answer a simple question? If the deuterocanonical books Luther said, in agreement with the OT church and much of the ancient NT church, were outside of the canon, if they were - can you operate with that for a moment on a purely theoretical level? - precisely which doctrines of the Holy Scriptures would be left unrepresented in the remaining canon?
As an example, let us that any one of the four gospels were left out of the canon - remember, this is theoretical, on one is advocating this!!! - which doctrine(s) of the Holy Scriptures would we lack support for in what remains?
Is that Better? Luther was a dolt if not a heretic and I don't see how anyone can be shocked by another religion changing their canon if they're part of any sort of Protestant or Protestant derived religion, including "Independent my own Pope" ones. Sure, Mormons wrote them self a brand new book, but Luther changed the interpretation and canon of existing Scripture so much he may as well have just done the John Smith. Even Calvin wrote a book he claimed should be included in the canon as being equal to anything the Apostles wrote, so far from being a big brave guy, Luther was just a horny drunk who couldn't get his own head around Grace. There, no drive by.
Now, exactly what's the big deal about Mormons doing far less than Luther the dolt did? And I ask that having spent the majority of my life a devout Lutheran. Right up until I found and read as many of his own writings as I could find rather than accepting the BS about him. Anyone who reads all of what Luther and Calvin wrote will either be some flavor of Catholic by the time they're finished, or never believed in anything but their own intellect to begin with.
JMHO
have a nice day