While it is true that St. Robert Bellarmine thought it impossible that a pope could ever lose the faith and hence the papacy, he considered this opinion not theologically certain. For this reason he proceeded to examine the question of what would happen were a pope to become a heretic.
"The fifth opinion (regarding a heretical pope) therefore is true; a pope who is a manifest heretic by that fact (per se) ceases to be pope and head (of the Church), just as he by that fact ceases to be a Christian (sic) and a member of the body of the Church. This is the judgment of all the early fathers, who teach that manifest heretics immediately lose all jurisdiction."
-St. Robert Bellarmine, Church Doctor, De Romano Pontifice, ch. xxx.
"The faith is necessary for me to such an extent that, having God as my only judge in other sins, I could however be judged by the Church for sins I might commit in matters of faith."
-Pope Innocent III, Billot, Tract. de Ecclesia Christi, p. 610.
Any pope who "wished to overturn the rites of the Church based on Apostolic Tradition" would become a schismatic, not to be obeyed.
-Francisco Suarez, S.J., (1548-1617), "Most Exalted and Pius Doctor",
De Charitate, Disputatio XII de Schismate, sectio 1
"All offices shall be vacant ipso facto (without a declaration required) by tacit resignation ... #4 by public defection from the Catholic Faith."
1917 Code of Canon Law, Canon 188, no. 4.