Pope Stephen III
Originally, lay status did not bar election to the See of Rome. Bishops of dioceses were sometimes elected while still catechumens, such as the case of St. Ambrose, who became Bishop of Milan in 374. In the wake of the violent dispute over the 767 election of Antipope Constantine II, Pope Stephen III held the synod of 769, which decreed that only a cardinal priest or cardinal deacon could be elected, specifically excluding those that are already bishops. Church practice, however, deviated from this rule as early as 817 and fully ignored it from 882 with the election of Pope Marinus I, the Bishop of Caere. Nicholas II, in the synod of 1059, formally codified existing practice by decreeing that preference was to be given to the clergy of Rome, but leaving the cardinal bishops free to select a cleric from elsewhere if they so decided. The Council of 1179 rescinded these restrictions on eligibility. On 15 February 1559, Paul IV issued the Papal Bull "Cum ex apostolatus officio", a codification 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.