"This principle is most certain. The non-Christian cannot in any way be Pope, as Cajetan himself admits (ib. c. 26). The reason for this is that he cannot be head of what he is not a member; now he who is not a Christian is not a member of the Church, and a manifest heretic is not a Christian, as is clearly taught by St. Cyprian (lib. 4, epist. 2), St. Athanasius (Scr. 2 cont. Arian.), St. Augustine (lib. de great. Christ. cap. 20), St. Jerome (contra Lucifer.) and others; therefore the manifest heretic cannot be Pope."
St. Robert Bellarmine, Doctor (i.e. "Teacher") of the Universal Church
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