We know exactly why Protestants won't accept it. Do you think Protestants were the first group of rebelious heretics to spin-off and declare themselves "the Church?"
The middle east was full of them a thousand years ago....of course, now they're all Muslims, but that could never happen to you.
Individual believers makeup the body of Christ. It's an organism, not an organization and no believer thinks that the local denomination they attend is the real *church*.
Catholics say that you have to be Catholic to be saved.
Believers know you need to be believers in Jesus to be saved regardless of denomination.
There is a mixture of saved and unsaved in every local assembly because it's not baptism, church membership, denominational affiliation which saves.
It's faith in Jesus which transcends denominational lines.