This is probably the fundamental difference between Orthodoxy and Protestantism. Protestants believe that those who ultimately are saved (which is roughly equivalent to theosis, but theosis seems to be a more elegant term) are by definition members of the Church, they therefore believe that the bounds of the Church are not well-defined.
In the end, however, we come to the same conclusion - it doesn't matter which Church's rolls your name is inscribed in; rather, it is theosis.
(As an aside, I had to look up theosis. Interesting concept. I should examine it further.)
Excellent summary, in my view.