Consider the question, Why not?
"Catholics do not believe that Protestants who are baptized, who lead a good life, love God and their neighbor, and are blamelessly ignorant of the just claims of the Catholic Religion to be the one true Religion (which is called being in good faith), are excluded from Heaven, provided they believe that there is one God in three Divine Persons; *that God will duly reward the good and punish the wicked; that Jesus Christ is the Son of God made man, who redeemed us, and in whom we must trust for our salvation; and provided they thoroughly repent of having ever, by their sins, offended God.
Catholics hold that Protestants who have these dispositions, and who have no suspicion of their religion being false, and no means to discover, or fail in their honest endeavors to discover, the true Religion, and who are so disposed in their heart that they would at any cost embrace the Roman Catholic Religion if they knew it to be the true one, are Catholics in spirit and in some sense within the Catholic Church, without themselves knowing it.
She holds that these Christians belong to, and are united to the "soul," as it is called, of the Catholic Church, although they are not united to the visible body of the Church by external communion with her, and by the outward profession of her faith."
-- Catholic Belief: A Short and Simple Exposition of Catholic Doctrine, Chapter XLIV. (NY: Benzinger Brothers, 1884)
ONE Lord, ONE Baptism, ONE Faith, ONE Body - The Lord Our God Is ONE!