Do not get me wrong. I am not saying that WW II and the Holocaust were the only reasons for a change in the salvation teaching. Obviously, we all know that Modernism generally was a big factor. Undoubtedly many in the Church by the mid 1900's had begun to doubt the very narrow view of salvation that was taught by the Papal Bull Cantate Domino. Still, I just have a feeling that the horrendous pain and suffering that humanity endured in WW II and the Holocaust, gave a real push to the Church leaders to respond by loosening the doors of salvation, so to speak.
I guess I am saying that when the Church leaders looked at the terrors of WW II and the Holocaust, they asked themselves a question. With millions of Jews and Protestants and Muslims having suffered and died, did God automatically condemn all of them to eternal punishment, just because they were not members of the visible Catholic Church? Somehow, I believe that their hearts were softened by the awesome suffering of humanity and they decided it was time to express officially what most of them already believed, that the exclusivist position of Cantate Domino was no longer tenable in the mid-1900's.