It’s self-authenticating in that the Lord authenticates it to those whom He is saving. Only He can give us understanding. John 6:45 teaches that all those being saved are “taught of God.”
No it isn't, though Protestants have to insist that it is. And the radical Protestant notion that G-d individually teaches each "saved" person is perfect in theory but absolutely unworkable in practice . . . unless G-d is teaching all those millions of people who disagree about so many issues different things.
Only G-d Himself is self-authenticating. If the Bible were self-authenticating, the Bible would (chas vechalilah!) be G-d.
At a certain point in history G-d authorized the Bible. But it wasn't two thousand years ago. It was some 3300 years ago when He spoke audibly to some three million people. That is how we know the Torah is from Heaven. But of course this didn't happen to chrstians so Protestants have to invoke the Bible as self-authenticating while the Catholics/Orthodox have to invoke the church as self-authenticating. One is bibliolatry, the other is ecclesiolatry.
The "new testament" which you are invoking was added to the bible by the historical Catholic/Orthodox church you and I despise so much. But thanks for playing.