The Church teaches that once someone is Baptised, an indelible mark is made on their soul and that makes the person Catholic forever, regardless of how they feel about it, regardless of whether or not they agreed to it, etc. If the indelible mark of Baptism exists on the person, then that person is claimed by the Church as Catholic. This is a teaching of the Church.
Let me tell you why I doubt that very much. Consider a man baptized in the Eastern Orthodox Church, a schismatic church. The Catholic Church considers their sacraments, including baptism, as valid, in spite of the fact that the Eastern Orthodox Church is schismatic. Now, when this man is baptized in the Eastern Orthodox Church, is he then considered a Catholic just because he received a valid baptism? I don’t think so. He becomes a Christian because of that baptism, yes, and that leaves an indelible mark. But he remains a non-Catholic. To become a Catholic, or a member of the visible Catholic Church, he still has to make the necessary steps to be in communion with the Catholic Church, but he does not need to be baptized again because his original baptism was valid.
Suppose now that he was admitted into the Catholic Church and becomes a Catholic, but later decides to leave the Catholic Church and was excommunicated. In that unfortunate event, he loses his status as a member of the Catholic Church, is it not? So, it is not true to say that “once a Catholic, always a Catholic.”
Now, suppose that this man, instead of being baptized in the Eastern Orthodox Church, is baptized in the Catholic Church. In that case he does not merely become a Christian, but he also becomes automatically a bona fide member of the Catholic Church. But this does not mean that he cannot lose this status of being a Catholic. If he leaves the Church, for example, and becomes a protestant, then I think he ceases to be a Catholic; that means, he ceases to be a member in good standing of that ecclesiastical body known as “The Catholic Church.”
Whenever I speak of the “Catholic Church,” I am referring only to the
visible Church of Christ on earth. Therefore, do not confuse it with the “mystical Body of Christ,” or the greater “Church of Christ.” The Church of Christ, or the Mystical Body of Christ, has a far wider membership. It includes the
Church Triumphant (consisting of the saints in heaven), the
Church Suffering (consisting of the souls in purgatory), and the
Church Militant (which consists of those members of the mystical Body that are still on earth). The Church Militant, in turn, consists of the visible Catholic Church (of which the Roman Pontiff is the visible Head), and those non-catholics who, though not baptized in the Catholic Church through no fault of their own, are living out virtuous lives, and who accept Christ as their Lord and Savior, and who are therefore regarded as “invisible members” also of the One True Church of Christ.
Going back to the question posted by the original OP, I say that if a Catholic honestly believes that the Catholic Church is not the true Church of Christ, and leaves, then he ceases to be a Catholic - that means, he ceases to be a member of the visible Catholic Church. But if he is honestly in error, and he is not guilty of negligence in the pursuit of the truth, and provided he still accepts Christ as his Lord and Savior, then he could still be considered an “invisible member,” or part, of the Mystical Body (or Church) of Christ.
Of course, all the above is just my opinion. If you think that I have been in error somewhere, please indicate a reliable source that we can both fall back on to resolve the issue. I am not a priest or a canon lawyer, just a plain catholic like most of us in this forum. Please don’t be mad when I disagree. You, too, are most welcome to disagree with me. We can agree to disagree.
God bless!