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Here is my take on your question. We need to take into consideration all four of these truths:
- Grace is what is necessary for salvation. All grace comes from God and through Jesus Christ.
- The Church is the body of Christ. Therefore all grace comes through the Head (Jesus) and His body (the Church).
- All who are validly baptized (from any denomination) enter the Church.
- The Catholic Church is the Church that Jesus historically founded; and the fullness of the Truth remains with her.
Putting these together we can deduce two conclusions:
- Catholics (as well as Orthodox) possess the best means to salvation. It is best for everyone to be Catholic.
- It seems possible for non-Catholics and non-Christians to attain salvation, if:
- God works outside of his sacraments for those that are invincibly ignorant, through no fault of their own, that Catholicism is the true faith, and
- Those that are invincibly ignorant are open to grace and follow God according to their conscience.
That may seem like a convoluted answer because Jesus did die for all but we also have to follow everything Jesus taught us to attain salvation.
For those that know the Catholic Church is the one Christ established, and freely refuse to enter it, they shall be judged harshly — they can't be saved (CCC 846).
For all that don't know, we hope and pray for their salvation through extra-sacramental grace that mysteriously flow through the Body of Christ to the world. In the meantime, Catholics have a responsibility to evangelize and catechize.