Sergios 53 was son of Christophoros 26: Lib. Pont. 96. 5, 97. 5. He was a sacellarius in 768 and later became secundicerius ("Sergio tunc sacellario; Sergius dudum quidem sacellarius, postmodum secundicerius"): Lib.Pont. 96. 5. He became secundicerius and also nomenculator early in the pontificate of Stephen IV (III) (Stephanos 84): Lib. Pont. 96. 16 (cited below). Sergios 53 and his father were troubled by the improper election as pope of Konstantinos 140; they obtained permission to become monks and set off for the monastery of the Saviour (near Reate, close to Spoletium); the abbot was all prepared to welcome them, when they diverged from the route and went to Spoletium, where they asked the Lombard dux Theodicius (Theodikios 1) to escort them to king Desiderius (Desiderios 3) at the Lombard court, north of the Po; Theodikios 1 agreed; they reached the king's court and asked him to help the Church against Konstantinos 140: Lib. Pont. 96. 5. On the date, see Duchesne, Lib. Pont., pp. 480-481, n. 3.

When allowed to leave court, they went to Reate, and then Sergios 53, accompanied by Waldipert 1 and supporters from Reate and Furcona as well as Lombards from the duchy of Spoletium, made their way to Rome, arriving on 29 July 768 and occupying the Pons Salaria; on the following day they crossed by the Milvian Bridge and came first to the Gate of St Peter and then the Gate of St Pancratius, where they were admitted by relatives of Christophoros 26 and Sergios 53; once in Rome, they made camp on the Janiculum out of fear of the Roman people: Lib. Pont. 96. 7-8. See also Racipert 1 and Toto 1.

After the overthrow of Konstantinos 140, Waldipert 1 had a certain Philippus (Philippos 11) made bishop of Rome, without the knowledge of Sergios 53; subsequently Christophoros 26 arrived, had Philippos 11 removed and Stephanos 84 chosen as bishop of Rome: Lib. Pont. 96. 10-11.

At the start of his pontificate, Stephen IV (III) (Stephanos 84) sent Sergios 53, now secundicerius and nomenculator ("Sergium antedictum, secundicerium et nomenculatorem illo in tempore existentem") to Francia to ask the Frankish rulers Pepin, Charlemagne and Carloman (Pepin 1, Karoulos 1 and Karoulomannos 2) to send learned bishops for a council in Rome: Lib. Pont. 96. 16. On arrival Sergios 53 found that Pepin 1 had died, but he was welcomed by the new rulers Karoulos 1 and Karoulomannos 2 and they readily granted the pope's request: Lib. Pont. 96. 17. Later Sergios 53 and Christophoros 26 encouraged pope Stephen IV (III) (Stephanos 84) to write to Karoulos 1 and Karoulomannos 2 for help in forcing the Lombards and their king Desiderios 3 to fulfil his promise and restore the rights ("iustitiae") of the Church which they had taken away ("imminentibus atque decertantibus in hoc sepius nominatis Christoforo primicerio et Sergio secundicerio"); his action roused the anger of Desiderios 3 against them and he plotted their downfall with the aid of Paulus Afiarta (Paulos 51); when they heard that the Lombard king was coming to Rome against them, they gathered a force from Tuscia and Campania and the duchy of Perusia and prepared to defend Rome: Lib. Pont. 96. 28. They and their followers assembled in protest at the plots against them, but after the pope had sent a message, via Andreas 31 and Iordanes 3, that to save themselves they should either go to a monastery or come to him at St Peter's, their support melted away (including that of Gratiosos 3, a relative of Sergios 53) and both surrendered, first Sergios 53, who fled by night to St Peter's and was arrested by Lombard guards and taken before Desiderios 3, and then Christophoros 26; pope Stephen IV (III) (Stephanos 84) tried to save them by making them both monks, but they were both taken from St Peter's by night by Paulos 51 and Desiderios 3 and had their eyes put out; Sergios 53 was then taken first to the monastery of Clivus Scauri and then to a cell in the Lateran palace, where he lived on, dying shortly before Stephanos 84 himself: Lib. Pont. 96. 29-32. Stephanos 84 described to his eventual successor, Hadrian I (Hadrianos 1), how Desiderios 3 had persistently broken his word and had blinded two nobles of the church, Christophoros 26 and Sergios 53 ("erui fecit oculos Christophori primicerii et Sergii secundicerii filii eius, suamque voluntatem de ipsis duobus proceribus ecclesiae explevit"); when he later complained to Desiderios 3, the Lombard king said that he should be grateful to him for removing these obstacles from his path: Lib. Pont. 97. 5.

An enquiry into the death of Sergios 53 ("antefati Sergii secundicerii mortem") held under pope Hadrian I (Hadrianos 1) found that he had been removed from his cell eight days before the death of Stephanos 84 by Calventzulus 1, on the orders of Paulos 51, Gregorios 77, Ioannes 239 (brother of Stephanos 84) and Calvulus 1, and handed over to the Campanians Lunisso (Lounisso 1) and Leonatius (Leonatios 1), who killed him; they showed servants of Hadrianos 1 where they had killed and buried Sergios 53, near a painted arch not far from Santa Maria Maggiore; his body was recovered and evidence found that he had been strangled and buried still half-alive: Lib. Pont. 97. 11 (cf. Anonymus 263). The bodies of Sergios 53 and his father were then buried with honour by Hadrianos 1 in St Peter's: Lib. Pont. 97. 14.

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