The Catechism of the Catholic Church defines these three sins against the faith in this way: 2089 Incredulity is the neglect of revealed truth or the willful refusal to assent to it.
"Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same…
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.
Finally, the person who refuses submission to the Roman Pontiff, whom Vatican I defined as having a universal primacy of authority over the whole Church, is at least a material schismatic. It was thus common in the past to speak of the schismatic Orthodox Churches who broke with Rome in 1054. As with heresy, we no longer assume the moral culpability of those who belong to Churches in schism from Rome, and thus no long refer to them as schismatics.
When it comes to Catholics who are formally guilty of heresy, apostasy or schism, the Church applies the penalty of excommunication. The 1983 Code of Canon Law, repeating the sanctions of the earlier 1917 Code, states,
- With due regard for can. 194, part 1, n. 2, an apostate from the faith, a heretic or a schismatic incurs automatic (latae sententiae) excommunication and if a cleric, he can also be punished by the penalties mentioned in can. 1336, part 1, nn. 1, 2, and 3.
- If long lasting contumacy or the seriousness of scandal warrants it, other penalties can be added including dismissal from the clerical state.
This canon is saying that once a person willingly repudiates Christ, embraces a heresy, knowing it to be contrary to divine and Catholic faith, or refuses submission to the Roman Pontiff , by virtue of the law itself they are automatically excommunicated. No ecclesiastical act is necessary and no public notice.
However, to incur this latae sententia excommunication one must satisfy the general conditions for canonical culpability set out in the Code. For example, a person who has not been diligent (prudently weighing the issues involved) in their action is not punished.
1. No one is punished unless the external violation of a law or a precept committed by the person is seriously imputable to that person by reason of malice or culpability.
2. A person who has deliberately violated a law or a precept is bound by the penalty stated in the law or that precept; unless a law or a precept provides otherwise, a person who has violated that law or that precept through a lack of necessary diligence is not punished.
3. Unless it is otherwise evident, imputability is presumed whenever an external violation has occurred.
A person who lacks the proper use of reason is likewise not punishable.
c. 1322 Persons who habitually lack the use of reason are considered incapable of an offense even if they have violated a law or a precept while appearing to be sane.
The following canon completes the list of conditions that can prevent the application of an excommunication and other ecclesiastical sanctions.
The following are not subject to penalties when they have violated a law or precept:
(1) a person who has not yet completed the sixteenth year of age;
(2) a person who without any fault was unaware of violating a law or precept; however, inadvertence and error are equivalent to ignorance;
(3) a person who acted out of physical force or in virtue of a mere accident which could neither be foreseen nor prevented when foreseen;
(4) a person who acted out of grave fear, even if only relatively grave, or out of necessity or out of serious inconvenience unless the act is intrinsically evil or verges on harm to souls;
(5) a person who for the sake of legitimate self-defense or defense of another acted against an unjust aggressor with due moderation;
(6) a person who lacked the use of reason with due regard for the prescriptions of cann. 1324, part 1, n. 2 and 1325;
(7) a person who without any fault felt that the circumstances in nn. 4 or 5 were verified.
To address your question specifically, are Catholics today who reject Catholicism and become protestant Outside of the Church? The answer is yes assuming that the former Catholic was diligent in his/her decision to reject the Church since that person would formally be a heretic and thus excommunicated.