V.R.S.

" To read your letter, one seriously wonders if you still believe that the visible Church whose seat is at Rome is indeed the Church of Our Lord Jesus Christ"
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Oh, they could rightly say: yes but it would not have been tantamount to saying they still believed that Ratzinger was the visible head of the Church of Our Lord Jesus Christ.
For example, one can reasonably believe that the visible Church whose seat is at Rome is indeed the Church of Our Lord Jesus Christ but the see happens to be vacant at the moment.