Because Traditional Catholic Teaching states that the Church is already united in the Catholic Church.
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This would have to be explained a great deal more.
also, NO ONE is stating that Catholic doctrine be changed to accommodate non-Catholics who wish to enter the Church. The entire force of the new evangelization being called for by the Church reinforces that strongly....and stresses that even baptized but not adequately catechized Catholics must learn exactly what the Church teaches and why!
I see both of you have yet to read Mortalium Animos.