The following is an excerpt from Catholic Belief: or A Short and Simple Exposition of Catholic Doctrine, by the Very Rev. Joseph Faa Di Bruno, D.D.
Catholics do not believe that Protestants [or other non-Catholic Christians] 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.
Very different is the case of a person who, having the opportunity, neglects to learn from genuine trustworthy sources what the Catholic Religion is and really teaches, fearing, that were he to become convinced of the truth of the Catholic Faith, he would be compelled by his conscience to forsake his own religion, and bear the worldly inconveniences attached to this step. This very fear shows a want of good faith, and that he is not in that insurmountable ignorance which could excuse him in the sight of God, but that he is one of those of whom it is said in Psalm xxxv. 4, "He would not understand that he might do well."
Fairness, no less than common sense, teaches that a man should study and examine the teaching of the Catholic Church from Catholic sources before condemning her. Surely no man ought to reject Catholic doctrine if he has not made himself well acquainted with them. Nor is it fair to form a judgment from misrepresentations made by ill-informed, interested, or prejudiced persons; one should rather, by the study of authorized Catholic works, judge of the truth with that calm and unprejudiced mind which the all-important subject of Religion deserves. Thus having heard both sides, you will be in a state to pass a right judgment and not in danger of being misled by prejudice.