Again, other than a small minority of the Anabaptists--the fringe groups of the Reformation, none of the Protestant Reformers believed that, and nor do the great majority of grateful Protestant Christians up to the present day.
Mainstream Protestants have never claimed membership in their bodies was a requirement for salvation, personal faith in Christ is...whereas, as evidenced here: "You are hell bound, generally speaking, yes, by the fact of your being separated brethren." (written above by a traditional RC), mainstream Romanists do.
Protestants with an understanding of the invisible Church, headed up only by Jesus Himself, are far more CATHOLIC (or universal, the meaning of the word) in their beliefs on who is a Christian, than the Church of Rome has ever been.
One of the interesting outs of Protestantism is the lightning fast ability to cut other Protestants lose. Having fractured so many times, it is easy to simply disown other Protestants as "not mainstream" etc. The RCC owns every word ever spoken within the Church, but Protestants only take ownership of what their current minister said last week. That can have its down sides.
I remember well the minister at our small country church growing up. He despised Catholics and once brought in missionaries he wanted to pitch funding for. They were "working to convert Catholics to Christianity". Bless my father who indignantly stated that he'd never heard anything so ridiculous as converting Christians to Christianity.
Again, other than a small minority of the Anabaptists--the fringe groups of the Reformation, none of the Protestant Reformers believed that, and nor do the great majority of grateful Protestant Christians up to the present day.
I think you greatly underestimate the size of that minority and also forget the vicious anti-catholicism of that first generation (the wars were brutal), but I agree its now a minority.
I'm an adult convert to the RCC and I have to tell you that I find the steadfastness of the RCC very comforting. It is made up of men who are sinners like me, but as an institution it is a rock. The RCC will not be performing gay marriages or accepting abortion. Do I wish the church would excommunicate certain false Catholics? Yes.
None of us should beat each other with the sins of our fathers, nor should we look for reasons for division. We have our disagreements, but serve the same master. I think some Protestants are feeling that they have much more in common with the RCC than they do with other Protestants. Not surprising given that "Protestant" is such a catch all term.
IMHO the more conservative Protestant churches, the RCC, and the Orthodox churches should form an alliance of purpose to defend and promote Christian moral values.