While separation from the Catholic Church is an obstacle for those seeking Heaven, that does not necessarily mean that every person who denies the Catholic Church will go to Hell.
In his book What Catholics Really Believe: 52 Answers To Common Misconceptions About the Catholic Faith, Karl Keating answers this tough question.
He begins by citing Pope Pius XII’s 1943 encyclical Mystici Corporis Christi, translated to On the Mystical Body of Christ. This encyclical explains three aspects identifying a Catholic: valid baptism, participation in communion with the Church, and profession of the Faith.
Once someone is baptized, they are sealed with the gift of the Holy Spirit, and original sin is washed away from their soul permanently. A person cannot be “unbaptized.” However, no one is forced to remain in the Catholic Church. Those who willingly disassociate themselves with the Church are no longer members.
Christians reject or leave the Catholic Church for many reasons—some have good intentions, some don’t. Sometimes people leave the Church and join a different Christian denomination for what they believe are good reasons. In other words, these Christians are more misguided than unfaithful. God alone can judge, for He alone knows their intentions—we are simply called to evangelize to these souls lovingly and truthfully.
Karl Keating explains how leaving the Church in good faith can affect one’s justification:
“[These Christians] remain related to the Church spiritually, even though they cease to be legal members of it. They still may achieve justification and salvation, but these are harder to achieve the further one distances oneself from the complete truth, found only in the Catholic Church, and the ordinary sources of grace, the sacraments. Leaving the Church, even with the best of intentions, is a great blunder because, all things being equal, the move diminishes one’s chances for heaven.”
However, if one leaves the Church in bad faith—knowing that Catholicism was founded by Jesus Christ and holds the ultimate truth—that person has knowingly abandoned the truth and would not enter Heaven.
At the end of the day, however, we as human beings cannot judge the interior state of someone’s soul. The best thing we can do is pray for their salvation.
For more answers to tough questions that Catholics face, check out Karl Keating’s What Catholics Really Believe.