“In your opinion, is this statement of St Robert Bellarmine infallible?”
Absolutely:
Due to the fact that his statement agrees with that of the infallible doctrine of the Church as taught by Popes. This is based on the following (see http://www.holywar.org/Ratzinger.htm):
“If anyone holds to one single one of these (heresies) he is not a Catholic.”
— Pope Leo XIII (from Encyclical Satis Cognitum:28)
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What is Heresy?
“Heresy consists in a stubborn denial of truths which have been defined and proposed by the Church as divinely revealed doctrines.” (Canon 1324-1325 of the 1917 Code of Canon Law).
What must we believe?
“By the divine and Catholic Faith, all those things must be believed which are contained in the written Word of God and in tradition, and those which are proposed by the Church, either in a solemn pronouncement or in her ordinary and universal magisterium, to be believed as divinely revealed.” (Vatican Council I, Denzinger 1792)
Who does not believe all those things taught by the Magisterium of the Church?
“Any baptized person who … obstinately denies or doubts any of the truths proposed for belief by divine and Catholic faith, is a heretic.”(C. 1325)
Can a heretic be a valid Pope of the Roman Catholic Church?
No. The Papal Bull Cum ex apostolatus officio of Pope Paul IV teaches that: if anyone was a heretic before the Papal election, he could not be a valid pope, even if he is elected unanimously by the Cardinals. Canon 188.4 (1917 Code of Canon Law) teachers that : if a cleric (pope, bishop, etc.) becomes a heretic, he loses his office without any declaration by operation of law. St. Robert Bellarmine, St. Antonius, St. Francis deSales, St. Alphonsus Liguori, and many other tehologians all teach that a heretic cannot be a pope: “If however, God were to permit a pope to become a notoriously and contumacious heretic he would by such fact cease to be pope, and the apostolic chair would be vacant.” — St. Alphonsus Liguori, Church Doctor: Verita bella Fede. Pt. iii, Ch.viii, 9-10.
His resistance to doctrine is invariably couched in elision and ambiguity rather than declarative refusal.
This makes Pope Francis comments no less dangerous; in fact, I would say MORE dangerous, since it always allows him to keep on sowing confusion while maintaining a certain plausible deniability.