The Church's moral theology has always distinguished between objective or material sin and formal sin. The person who holds something contrary to the Catholic faith is materially a heretic. They possess the matter of heresy, theological error. Thus, prior to the Second Vatican Council it was quite common to speak of non-Catholic Christians as heretics, since many of their doctrines are objectively contrary to Catholic teaching. This theological distinction remains true, though in keeping with the pastoral charity of the Council today we use the term heretic only to describe those who willingly embrace what they know to be contrary to revealed truth. Such persons are formally (in their conscience before God) guilty of heresy. Thus, the person who is objectively in heresy is not formally guilty of heresy if 1) their ignorance of the truth is due to their upbringing in a particular religious tradition (to which they may even be scrupulously faithful), and 2) they are not morally responsible for their ignorance of the truth. This is the principle of invincible ignorance, which Catholic theology has always recognized as excusing before God.
The same is true of apostasy. The person who leaves not just the Catholic Church but who abandons Christ Himself is materially an apostate. He is formally an apostate through willful, and therefore culpable, repudiation of the Christian faith.
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