(29 Jan. 904–Sept. 911)
A Roman of aristocratic birth, the son of Benedict, made deacon by Stephen V, he was consecrated bishop of Caere, i.e. Cerveteri (unwillingly, he was to allege), by Formosus, took part in the ‘cadaver synod’ presided over by Stephen VI, and, when it posthumously condemned Formosus and annulled his ordinations, gladly regarded himself as reduced to the diaconate and accepted reordination as priest by Stephen VI; ambitious for the papacy, he did not wish the fact that he was a bishop to exclude him. A virulent hater of Formosus, he was elected pope by the anti-Formosan faction on the death of Theodore II (Dec. 897) and was even installed in the Lateran, but had to give way to the pro-Formosan John IX, who had the support of Emperor Lambert of Spoleto (d. 898). Deposed, condemned, driven into exile, he had his chance seven years later when the Formosan party in Rome split and the priest Christopher overthrew and imprisoned Leo V. Aided by Duke Alberic I of Spoleto (d. c.925), Sergius marched on Rome with an armed force, threw Christopher into gaol, was acclaimed pope, and was consecrated on 29 Jan. 904. Not long after, moved, it was said, by pity, he had Leo V and Christopher strangled in prison.
Sergius dated his reign from his original abortive election in Dec. 897, treating all his predecessors from John IX as intruders. In order to undo their work he immediately press-ganged the clergy by threats and violence to attend a synod which overturned John IX's Ravenna synod of 898 (it had excommunicated him, if, as seems likely, he was the Sergius mentioned), reaffirmed the ‘cadaver synod's’ condemnation of Formosus, and once again declared null and void the orders he had conferred during his ‘usurpation’. As Formosus had created many bishops, who in turn had ordained numerous clergy, the resulting confusion was indescribable. Sergius insisted that those whose orders were annulled should be ordained afresh; his policies were carried out with such threats and violence that few had the courage to resist. One who did resist was the Frankish priest Auxilius (c. 870–c. 930), whose acute and hard-hitting pamphlets defending the ordinations of Formosus (also of Stephen of Naples, another bishop who had moved sees) provide invaluable information about the controversy. His protests found echoes elsewhere throughout Italy, but in Rome the scandalized opposition had to keep quiet. There Sergius had the support of the noble families, notably that of Theophylact (d. c.920), financial director of the holy see but also consul and commander of the militia, and his ambitious, determined wife, the senatrix Theodora (d. after 916). In the absence of an emperor this family effectively governed Rome, and Sergius enjoyed such intimacy with it that he was reputed to have had a son, the future pope John XI, by Theodora's 15-year-old daughter Marozia. His and his immediate successors' dependence on the family was complete, though little different from what was happening elsewhere in Europe as the nobility asserted its authority over the church. Marozia's marriage in 909 to Alberic of Spoleto (d. 924) had the effect of strengthening the nobility's power over the Roman church, but it also brought much-needed political stability to the city. Nonetheless the dominance of Theodora and then of her daughter Marozia caused the following decades to be castigated as the pornocracy of the holy see.