See post #15 by HangnJudge.
A statement was issued Ex Cathedra. So, is truth changeable, or is it not?
Well, gee, it’s like this: in 1302 A.D. the Catholic Church AND the Greek Church in schism (salvation in one still recognized by the other) were the Christian church.
The Church had by then experienced a number of heretical breakaways. Pope Boniface invalidated them with his statement.
BTW, there were no Protestants then.