His first act of incontrovertible HERESY.
Is it possible for a heretic to be Pope?
Yes.
Therefore, the true opinion is the fifth, according to which the Pope who is manifestly a heretic ceases by himself to be Pope and head, in the same way as he ceases to be a Christian and a member of the body of the Church; and for this reason he can be judged and punished by the Church. This is the opinion of all the ancient Fathers, who teach that manifest heretics immediately lose all jurisdiction, and outstandingly that of St. Cyprian (lib. 4, epist. 2) who speaks as follows of Novatian, who was Pope [i.e. antipope] in the schism which occurred during the pontificate of St. Cornelius: “He would not be able to retain the episcopate [i.e. of Rome], and, if he was made bishop before, he separated himself from the body of those who were, like him, bishops, and from the unity of the Church.”
According to what St. Cyprian affirms in this passage, even had Novatian been the true and legitimate Pope, he would have automatically fallen from the pontificate, if he separated himself from the Church.
St. Robert Bellarmine, Doctor of the Church