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Coat of arms of Pope Paul IV
Cum ex apostolatus officio is the name of a papal bull issued by Pope Paul IV on 15 February 1559 as a codification or explicitation of the ancient Catholic law that only Catholics can be elected Popes, to the exclusion of non-Catholics, including former Catholics who have become public and manifest heretics.
The immediate provocation was Pope Paul's suspicion that Cardinal Giovanni Morone, who was popular and expected to succeed him, was secretly a Protestant. Pope Paul IV believed that it was necessary to prevent or negate Morone's possible election as his successor. He wanted to set it in Church law that no manifest heretic can lawfully hold the Office of St. Peter.