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If this secret pagan publicly professes the Catholic faith I don't see why he wouldn't be a member of the Church. If he was baptized as an adult, but internally rejected this act, even at the very moment of it's carrying out, he would be a member, a dead member.

Is that what the rest of this thread is like?

The crisis in the Church can be summarized as an ecclesiological battleground.

Where is the source of his error?

Here is Bellarmine explicitly excluding pagans from membership in the Church:

Cantarella,

When you quote me it should always be done in context.  I attributed this source to Msgr. Joseph Fenton in an article in AER and that should be included in your question.  This citation says that St. Robert Bellarmine believed a non-baptized man pretending to be a Catholic, if accepted as such by a Catholic society, would be a member of the Church.
 
Msgr. Fenton wrote:

St. Robert was perfectly in line with a then existent theological tradition in holding that a man could he a true member of the Catholic Church when he possessed the outward bond of unity with the Church, apart altogether from the inward bond. He believed that a man was a real member of the Church when he had this outward bond of union, even though he had no true and inward Christian faith whatsoever.

He differed, however, from other ecclesiologists of his time in his concept of the outward bond itself. He held literally and consistently that these factors which were capable of making a man a member of the true Church and sufficient to constitute him such were the profession of the Christian faith and the communion or reception of the sacraments, under the direction of legitimate ecclesiastical pastors and ultimately, under the leadership of the Roman Pontiff. He definitely did not teach that the baptismal character was necessary for real membership in the Church.

He definitely taught that those who have not given their names to Christ through baptism, but who follow other religions, "are not members of the Church." He also denied that catechumens, those who were preparing for the reception of baptism, and thus for entrance into the true Church of Christ, were members of the kingdom of God on earth. But, in the light of what he has set down towards the end of the tenth chapter in his De ecclesia militante, it is obvious that he considered an unbaptized man a true member of the Church when that man lived in society as a Catholic and was accepted as such, either by reason of a mistake about his status, or because the man cold—bloodedly falsified his position, claiming to have been baptized when he knew well that he had never received the sacrament.

It is manifest that this particular aspect of St. Robert’s teaching is unacceptable in the light of Mystici corporis. It must he remembered, however, that St. Robert’s faulty description of the Church’s outward bond of unity in no way militates against his teaching about the possibility that occult heretics can be members of the true Church, and in no way invalidates the arguments he employed in favor of that contention.
Msgr. Joseph Fenton, AER, The Status of St. Robert Bellarmine's Teaching About the Membership of Occult Heretics in the Catholic Church; page 221

And answering your previous post.

Dogma is the proximate rule of faith and the definition of a heretic proves this necessarily.  You are either unwilling or unable to understand this.  No one can see this for you.  In a mathematics textbook, you will typically find  an axiom proposed followed by its mathematical proof.  A lot a students cannot see the proof, they just assume the book is correct, memorize the axiom and leave it at that, but is always better to understand the proof which becomes helpful in higher orders of study.
 
The definition of a heretic is a baptized person who rejects one or more dogmas.  He does not keep Dogma as his rule of faith.  This is the essential definition that provides the genus and species.  It proves that the faithful are those who keep Dogma as the rule of faith. 
 
It is worth repeating for the benefit of others even if you cannot appreciate it.  For you the question becomes why do S&Sers reject this evident truth?  I think it is because they end up in a church that has no pope, no magisterium, no dogmas and cannot be the Catholic Church.  If you turn your back on the revealed truth as your rule of faith, the current error is less psychologically troubling.
 
Do you know that the condemned error against Huss is of the same category of authority as the condemnations of Luther in Exsurge Domine?  They are not dogmatic truths and therefore must be contextualized.  The "foreknown" are those predestined to Hell.  Huss held that only those predestined to heaven were members of the Church.  A wicked pope in a state of mortal sin is still a material member of the Church but he has lost the life of sanctifying grace by definition. That is, he is no longer a temple of the Holy Ghost, an heir to heaven.  The Church is the Mystical Body of Christ and the Holy Ghost is the "Soul of the Church."  If any Catholic is without sanctifying grace he cannot be a living member of the Church.  Exactly what his relationship is admittedly a matter of theological dispute but as Fr. Fenton said that ultimately everyone is either a member of the Church or not.  If we are not perfectly accurate in discerning the grey areas it is our incomplete understanding of the problem.  You are free to speculate in these area of uncertainty but you are not free to engage in speculation and practical decisions that overthrow dogma. 
 
You are in a church that has no pope, no magisterium, no rule of faith and no material or instrumental means to ever correct these permanent defects.  You are in a church that is not Catholic and never will be.  Instead of trying to figure out how you ended up in this spiritual desert, you try only to overthrow the authority of dogma whenever it suits your purpose.

Drew