Protestantism is material heresy. The question is whether a Protestant who has converted to Catholicism can possibly be a valid Pope. But a related question has the same answer: can a Catholic heretic, who repents and returns to the fullness of faith, can then be a valid Pope.
Pope Paul IV issued a decree in 1559 with the following wording:
“Hence, by this Our Constitution which is to remain valid in perpetuity, in abomination of so great a crime (than which none in the Church of God can be greater or more pernicious) by the fullness of our Apostolic Power, We enact, determine, decree and define (since the aforesaid sentences, censures and penalties are to remain in efficacious force and strike all those whom they are intended to strike) that….” [Pope Paul IV, Ex Apostolatus Officio, n. 3.]
This decree goes on to issue an automatic penalty against Patriarchs, Cardinals, and Bishops, without any juridical proceedings under Canon Law, including excommunication, loss of all authority and privileges, loss of ability to participate in synods and conclaves, invalidity of their will and of any inheritance, and other penalties. The wording of a decree under the temporal authority of the Pope could hardly be more definitive. Pope Paul IV attempted to have this decree remain in force ‘in perpetuity’ and therefore he intended to bind future Popes to it. Yet this decree did not remain in force after the death of Pope Paul IV. Canon Law currently incorporates no such broad, severe, and automatic penalties, especially on Patriarchs, Cardinals, and Bishops.
More importantly, the same decree claims that a Roman Pontiff, who is elected even with the unanimous uncontested agreement of all the Cardinals, is not a valid Pope if he had ever been a heretic, or if he had ever deviated from the Catholic Faith at any time prior to his election. Again, this portion of the decree was issued with the strongest possible wording and was said to be perpetually binding and irrevocable.
Yet this same decree did not retain its force subsequently in the history of the Church. Successive Popes have not taught or held that a Pope is not a valid Pope if ever in his life, prior to his election, he deviated from the Catholic faith. The Apostolic Constitution of Pope Saint John Paul II, Universi Dominici Gregis, describes the conditions for validity of the Pope as three only:
(1) valid election,
(2) ordination to the Episcopal degree (as a Bishop),
(3) the willing acceptance of the office of Pope by the man who is elected.
Now the attempted decree by Pope Paul IV, if it had been accepted by subsequent Popes, would have cast a doubt over the validity of every subsequent Pope. How could anyone know if a man who is elected Pope (who may well be advanced in age) had ever deviated from the Catholic Faith in his entire life? And the decree gives no account to a man who deviates from the faith, but then repents and returns; his election is still said to be invalid.
I conclude that the decree of Pope Paul IV is not valid, despite its very strong wording, because it would effectively reduce the authority of each successive Pope, in contradiction to the establishment by Christ of an indefectible Church led by the successor to the Apostle Peter. From this example, we learn that the temporal authority of any Pope, no matter what he himself may claim, is limited even during his own reign, and is not necessarily binding, after his death, on subsequent Popes and on the Church as a whole.
When a Pope has died, the teaching authority of the Church does not die. The Cardinals and Bishops who remain cannot exercise the infallible teaching authority of the Magisterium; there can be no new infallible definitions of doctrine until the next Pope is elected. However, the Cardinals and Bishops, as true successors to the Apostles, can continue to teach under the Ordinary Magisterium, and can continue to teach any and all past teachings previously established as infallible.
When a Pope has died, the temporal authority of the Church does not die. The Cardinals and Bishops can continue to exercise the temporal authority of the Church; this authority is generally fallible, and so they do not have to wait for the next Pope to be elected in order to exercise this authority, especially in cases of grave necessity. Also, concerning the norms for the election of the Pope, such norms fall under the temporal authority of the Church. Therefore, when a Pope has died, the Cardinals and Bishops have the authority to deviate from the norms for electing the next Pope for grave reasons.
If one Pope could bind future Popes to his decisions of the prudential order, to his decisions in matters of discipline, then each successive Pope would have ever less authority. For each Pope would be bound by an ever-increasing set of past prudential judgments. Such a plan is contrary to the teaching of the Magisterium that each and every successor of Saint Peter the Apostle has the same authority as Peter. Therefore, one Pope cannot bind future Popes to his decisions on discipline.
On the other hand, teachings on faith, morals, and salvation are equally binding on all Popes (and on all the faithful). And so each Pope is bound by the infallible teachings of all his predecessors. For all are bound to hold and teach one and the same Sacred Deposit of Faith.
Therefore, can a man be elected as Roman Pontiff, validly, despite being a convert to Catholicism from Protestantism? Yes.
If you think otherwise, consider this: All the early Popes were converts. None was baptized as an infant and raised as a Catholic from childhood, for the Catholic Christian Faith had just begun. They all converted in adulthood. So a Pope need not have been a Catholic or a Christian for his entire life in order to be validly elected as the Roman Pontiff.
Therefore, also, a man can be elected as a valid Pope if at some time in the past he held an heretical view, then later repented.
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