Sergius’ ties with the family of Theophylact were made even closer, at least according to rumour, by Sergius’ supposed affair with Theophylact’s daughter, Marozia. This relationship was promoted by Marozia’s mother, Theodora, and the result of this affair was a male child who became Pope John XI (931–935).
The only source of this affair is the chronicler Liutprand of Cremona, writing some 50 years after the events of Sergius’ pontificate. Neither:
both exact contemporaries of Sergius, and both hostile towards Sergius for his attacks on Formosus, mention this allegation at all.
The affair, while not impossible, would not have persisted beyond Marozia’s marriage to Alberic I of Spoleto in 909.
The question of whether Theophylact and Theodora needed to tie Sergius to them by such means, particularly when Sergius was already deeply indebted to them for his elevation to the papacy, as well as wasting Marozia in a relationship when, as the daughter of an important house, she would have been a valuable tool to link via marriage to another noble house, is open to debate.
The birth of the future John XI in 910, after her marriage to Alberic, would seem to indicate that Sergius was not the father. However, it was highly unusual that the eldest son of a noble house would be destined for a career in the church, instead of inheriting his father’s title.
That the younger brother Alberic took his father’s place as duke of Spoleto, suggests the possibility that the elder brother John was illegitimate, with Sergius being the most likely candidate for his father.
The paternity of John XI is still a matter of dispute. His mother was certainly Marozia, the most powerful woman in Rome.
According to Liutprand of Cremona (Antapodosis, ii. c. 48) and the "Liber Pontificalis," his father was Pope Sergius III (904–911),
agree with Liutprand that Pope Sergius III fathered Pope John XI.
If that is true, John XI would be the only known illegitimate son of a Pope to have become Pope himself. (Silverius was the legitimate son of Pope Hormisdas).
On the other hand,
maintain that Pope John XI was sired by Alberic I of Spoleto, Count of Tusculum.
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