The other place where
Vatican II addresses the hypothetical tribesman is in Lumen Gentium, chapter 2. That clearly says that we are obligated to follow God as best we can. A tribesman who has no knowledge of the historical Jesus obviously can't become a Christian. (Incidentally, this is not a new idea--it goes back at least to the 4th century.)
However, even with that "out" for tribesman, atheists, and even Unitarians, there's a lot more to it than basically being an OK guy.