Wrong. If that were the case, then every suspect brought to trial would not be convicted unless he admitted he is contradicting a Church teaching. However, pertinacity can be demonstrated with moral certitude without the suspect admitting his heresy.
"Thus it is proven that according to Catholic Doctrine, for one to be considered a formal heretic, it is not necessary that he, 'renounce the Church as the RULE of faith by PUBLIC PROFESSION', or explicitly admit that one is knowingly in heresy; but it suffices that one either 1) assertively state his disbelief in one single article of faith, because, one who offends against even one article is guilty of all, because disbelieving in one destroys the formal cause of faith from which that article depends; or, that the nature or circuмstances of ones words or deeds constitute moral certitude of formal heresy. Hence, Cardinal Robert Bellarmine, who was a high official of the Roman Inquisition, and is a Doctor of the Church, teaches that, 'men are not bound, or able to read hearts; but when they see that someone is a heretic by his external works, they judge him to be a heretic pure and simple [simpliciter], and condemn him as a heretic.'"
Kramer, Paul. To deceive the elect: The catholic doctrine on the question of a heretical Pope . Kindle Edition.
It is true that there is one way for a heretic to be declared a formal heretic without admitting it himself. This happens when a proper authority confronts him (in a trial for example) and says, "I am telling you that what you are saying is heresy, and if you do not retract, you will become a formal heretic". The problem today is that the pope himself is a material heretic, and no man on Earth has the proper authority over him to confront him authoritatively. I think St. Robert Bellarmine in addressing this possibility teaches that in such a case, the college of bishops could act as that authority. Well, let them do so, and Francis will lose his office if he does not retract.
St. Robert's statement, quoted by Fr. Kramer, needs to be understood in the light of the distinction between objective and subjective. It is the same distinction that helps us reconcile in our minds two quotes of Our Lord about judging others.
"Beware of false prophets, who come to you in the clothing of sheep, but inwardly they are ravening wolves. By their fruits you shall know them". (Matthew 7:15-6). Our Lord teaches us to judge our pastors.
"Judge not, that you may not be judged," (Matthew 7:1). Our Lord teaches us not to judge.
The distinction is that we must judge our pastors objectively, and must not judge our pastors subjectively.
Objective--relating to the facts
Subjective--relating to the person
Objective judgement of Pope Francis: This man smiles a lot and has nice and sweet words, but his teaching does not match the dogmas of the Faith. Francis is a wolf. Let's stay clear.
Subjective judgement of Pope Francis: This man, in his heart, hates Jesus Christ, and intends to lead souls to Hell.
The objective judgement is commanded by Our Lord, the subjective judgement is forbidden. We cannot rule out the possibility that Francis really intends to work against Jesus Christ, but only God knows. All we know is that Francis is objectively a wolf, and we must stay away.
This is what St. Robert means when he says to condemn someone as a heretic "pure and simple". The facts show that he is a material heretic, the fruits show us he is an objective wolf, but whether or not it is his personal intention, in the courtroom of our minds, the jury is still out.