Why would you not know? The scripture you listed confirmed it. “For outside of the Christian church there is no truth, no Christ, no salvation.” Protestants are in a Christian Church so they are not outside the Church.
Any Christian Church that believes that their church is the only way to salvation is flawed and offensive. I would think a Catholic would be offended as well if a Protestant told them they were not sure Catholics could attain salvation (not my belief BTW).
We Catholics DO think something like the “fullness” of the Church “subsists”among those in communion with the Holy See.
But, while we loosely speak of “converts” the technically precise name for the act of a baptized person's “becoming a Catholic” is “being received into full communion.” It is quite wrong to say they ‘converted.’
If one can entertain the idea, “lex orandi lex credendi” then a good place to go for an experiential understanding of what we think would be the “solemn collects” of the Good Friday Liturgy.
In them, as I like to say it, we pray that God will, so to speak “apply” the benefits of the Cross to all the world. So we pray for Catholics, for all Christians, for Jews, for theists, and for atheists, that grace will be shed on them.
When I think of “the Church” in the broadest sense, I think of everyone we pray for on Good Friday.
There is no salvation outside the Church Christ founded. That Church is the Catholic Church (and the Orthodox but that discussion is outside this debate.)
Anyone who is validly baptized is baptized into the Church. Therefore they can attain salvation. I know how Catholics who lose the state of Grace they receive in baptism by mortal sin can be returned to a state of Grace. That is through sacramental confession. I do not know how non-Catholics who fall from the state of grace through mortal sin can return to a state of grace outside of the means Christ instituted in scripture, i.e., sacrental confession. I do know that those who die not in a state of grace cannot attain heaven so I do fear for the salvation of those outside the Catholic (and Orthodox) communion who commit grave sin.
To characterize basic Catholic belief as offensive is nothing new. What is new is a Catholic pope who is willing to once again state that outside the Church there is no salvation. We’ve lived with false ecumenism so long that non Catholics are shocked and offended now when they hear true ecumenism. In other words the problem isn’t that the pope said what he said, the problem is that what he said has been said so rarely these past 50 years due to false ecumenism.