Respectfully never read this before thanks for posting…
Sorry, had to cut your quote to fit my reply.
I think you should reflect on what Jesus said in John 17:21: “that they may all be one; even as thou, Father, art in me, and I in thee, that they also may be in us, so that the world may believe that thou hast sent me.”
Then in Matthew 16:18-19: "And I tell you, you are Peter, and on this rock I will build my church, and the powers of death shall not prevail against it. 19 I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.”
Further explained by Paul in 1 Corinthians 12:12-31, which I won’t quote because that’s quite long.
Essentially, all baptized are a part of the Body of Christ. His body is the Church, the true Church which he had founded on Peter, the Roman Catholic Church. Jesus wishes all of us to be united in the Church he founded.
Now for all those who had separated from the Roman Catholic Church, it is essentially separating from the Body of Christ. Salvation is understood to be how unified we are with Christ at the time of our deaths. For example, when protestant churches were formed, they still contain partial truths of the Catholic Church that keeps protestants still bound with the Body of Christ, but not completely. In order to completely be completely unified with the Body of Christ, each person must profess faith to the Church Jesus had founded on Peter, the Roman Catholic Church and all her teachings.
In correlation to the Cantate Domino, Pope Pius X did elaborate on it in the catechism in regards to having Full Knowledge to be a sin.
https://www.ewtn.com/library/CATECHSM/PIUSXCAT.HTM#Preliminary
Q . But if a man through no fault of his own is outside the Church, can he be saved?
A . If he is outside the Church through no fault of his, that is, if he is in good faith, and if he has received Baptism, or at least has the implicit desire of Baptism; and if, moreover, he sincerely seeks the truth and does God’s will as best he can such a man is indeed separated from the body of the Church, but is united to the soul of the Church and consequently is on the way of salvation
1 Timothy 3:15, Paul says that the Catholic Church is the “Pillar and Foundation of Truth”. Those that knowingly reject it, like Martin Luther had, will have their salvation compromised, but none of us will know Martin Luther’s judgement except God and those with the beatific vision. So if any protestants remain in the dark about the Catholic faith, their salvation may be in good standing, but if they do know and openly oppose it, then their salvation will be compromised.