One thing to keep in mind, that the Catechism sections already posted also go into, is that many groups of Protestants already have some type of association with the Catholic Church through the sacraments.
Trinitarian baptism is recognized by the Catholic Church, even if it is done by a Protestant minister in a Protestant church.
Marriage between two non-Catholics in a Protestant church is also recognized by the Church as sacramental if both of the people have been baptized.
So that’s two sacraments that a lot of Protestants already share with the Catholic Church.
In some cases, where for example the Protestant has married a Catholic in a Catholic ceremony, that Protestant has actually taken part in the Catholic sacrament itself.
So these people are part of the Catholic Church to some degree. It’s not as much as if they were converted Catholics, but they are not total outsiders.
Even Jewish people and Muslims who do not recognize Jesus as Savior have the possibility of salvation because they acknowledge the “God of Abraham” which is the same God that the Catholic Church acknowledges. See sections 836 to 856 of the Catechism.