Anastasios 25 is often referred to as Anastasius Bibliothecarius. He was the cardinal priest of the titulus of St Marcellus at Rome (presbiter cardinalis tituli beati Marcelli); at a Council held in Rome on 8 December 853 he was deposed and deprived of his priesthood because for the previous five years he had been absent from his parish in breach of canon law (eo quod parochiam suam per annos V contra canonum instituta deseruit) and had failed to attend two Councils to discuss the matter, even though summoned by letters from the bishop of Rome and by an embassy of three bishops (Ioannes 316, Nikolaos 48 and Petronakios 1); he had still failed to return to his church when the Life of Leo IV was composed (et in alienas usque hodie demoratur): Lib. Pont. 105. 92, cf. 90 (for the date), 106. 6 (he was deposed and anathematised), 106. 8 (properly excommunicated, deposed and anathematised at the synod by Leo IV (Leo 121)). In 855, after the death of Leo IV and the election of his successor Benedict III (Benediktos 7), a plot was formed by the bishop of Horta, Arsenios (Arsenios 3), to have Anastasios 25 made bishop of Rome; the envoys from the Frankish emperor Louis II (Lodoïchos 1), Adalbert 2 and Bernard 3, were persuaded to give him their support, and other supporters assembled at Horta (cf. Nikolaos 49, Merkourios 3, Gregorios 93, Christophoros 30, Agatho 9 and Radoald 1) with Anastasios and the imperial envoys; they then advanced on Rome: Lib. Pont. 106. 6-9. On his advice envoys sent from Rome by pope Benedict III (Bendiktos 7) were arrested and held in custody (see Georgios 167 and Maio 1): Lib. Pont. 106. 10. Near the basilica of St Leucius he and his followers met the bishops and clergy and the leading men of Rome and entered first the Leonine city, where he destroyed paintings and icons of Christ and the Virgin at St Peter's: Lib. Pont. 106. 11-12. He then entered Rome itself and broke into the Lateran palace, where he occupied the papal throne and had Benedict III (Benediktos 7) expelled and placed in custody: Lib. Pont. 106. 13-14. Attempts by the imperial envoys Adalbert 2 and Bernard 3 to terrorise the clergy and people into accepting Anastasios 25 failed, and so did their efforts to persuade the bishops of Ostia and Albanum to consecrate him: Lib. Pont. 106. 15-16. They finally gave way and agreed to the consecration of Benedict (Benediktos 7) and to the expulsion from the Lateran palace of Anastasios 25: Lib. Pont. 106. 17-18. Shortly afterwards the supporters of Anastasios 25 submitted to Benediktos 7: Lib. Pont. 106. 19.

He was the chief archivist of the Church at Rome (bibliothecarius sanctae Romanae ecclesiae; the Greek version calls him ὁ τῆς ἁγίας τῶν Ῥωμαίων ἐκκλησίας βιβλιοφύλαξ: (Mansi XII 1071) and flourished around the time of the patriarch Photios 1 and the Eighth Ecumenical Council (in 869); he translated into Latin the Acts of the Second Council of Nikaia (the Seventh Ecumenical Council); in the Acts of the second session he noted that the part of pope Hadrian I (Hadrianos 1)'s letter to the emperors that complained of the rapid elevation of Tarasios 1 from layman to patriarch, and which was therefore embarrassing to the easterners, had been omitted when the letter was translated and read out to the Council, and he therefore added the missing section to his Latin version: Mansi XII 1071-1074.

Addressee of a letter from Photios 1; he appears as a friend and supporter of Photios 1, with whom he had previously been in correspondence: Photius, Ep. 170 (II 45ff. Laourdas-Westerink) (written in c. 873 and addressed Ἀναστασίῳ πρεσβυτέρῳ καὶ βιβλιοθηκαρίῳ Ῥώμης).

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