Language scholar, Roman cardinal, and influential political counselor to 9th-century popes.

Related to an Italian bishop, Anastasius became cardinal priest of the Church of St. Marcellus, Rome, about 848, after gaining prominence as a Greek scholar. Deposed in 853 because of political activity, he stood for a short time as antipope to Benedict III (855-858). After a reconciliation Anastasius became papal librarian and disputed with the Greek Orthodox theologian Photius, patriarch of Constantinople (858-867; 878-886), over the question of the Holy Spirit's relationship within the Christian Trinity, a controversy crucial to Eastern and Western doctrinal differences leading to open schism.
Exhibiting thorough efficiency in expressing the ideas of the papacy, Anastasius maintained the post of librarian under Popes Adrian II (867-872) and John VIII (872-882). On the visit of Saints Cyril and Methodius to Rome, he supported their Christianizing mission among the Slavic peoples and their development of a native liturgy. Representing the Holy Roman emperor, the Frank Louis II (c. 824-875), Anastasius undertook a diplomatic mission to the Byzantine emperor Basil I (867-886) in an unsuccessful attempt to arrange a marriage between the two dynasties. He remained in Constantinople nevertheless to assist at the eighth general council of 869-870, which achieved final doctrinal formulations concerning the Trinity, emphasizing the divinity of the Holy Spirit and condemning Photian teaching. Anastasius' Latin translations of the council's proceedings and compilation of other documents relating to the monothelite controversy contributed to the history of Western theology. A later Latin collection also incorporated his "Threepart Chronicle" of Byzantine history from the 6th to the 9th century.
Included in Anastasius' major writings are commentaries on the influential 6th-century Neoplatonic philosopher Pseudo-Dionysius the Areopagite and probably the accounts of Popes Nicholas I and Adrian II in the Liber pontificalis (Latin: "The Book of the Popes"), an essential source for the history of primitive Christianity.