Acacius ( 7 ), patriarch of Constantinople, A.D. 471–489. Acacias was originally at the head of an orphanage at Constantinople, which he administered with conspicuous success (Suidas, s.v. ( Ἀκάκιος ). His abilities attracted the notice of the emperor Leo, over whom he obtained great influence by the arts of an accomplished courtier (Suidas, l.c. ). On the death of Gennadius (471) he was chosen bp. of Constantinople, and soon found himself involved in controversies, which lasted throughout his patriarchate, and ended in a schism of thirty-five years' duration between the churches of the East and West. On the one side he laboured to restore unity to Eastern Christendom, which was distracted by the varieties of opinion to which the Eutychian debates had given rise; and on the other to aggrandize the authority of his see by asserting its independence of Rome, and extending its influence over Alexandria and Antioch. In both respects he appears to have acted more in the spirit of a statesman than of a theologian; and in this relation the personal traits of liberality, courtliness, and ostentation, noticed by Suidas (l.c. ), are not without importance.

The first important measures of Acacius carried with them enthusiastic popular support and earned for him the praise of pope Simplicius. In conjunction with a Stylite monk, Daniel, he placed himself at the head of the opposition to the emperor Basiliscus, who, after usurping the empire of the East, had issued an encyclic letter in condemnation of the council of Chalcedon, and taken Timotheus Aelurus, the Monophysite patriarch of Alexandria, under his protection, A.D. 476. The resistance was completely successful. In the meantime Zeno, the fugitive emperor, reclaimed the throne which he had lost; and Basiliscus, after abject and vain concessions to the ecclesiastical power, was given up to him (as it is said) by Acacias, after he had taken sanctuary in his church, A.D. 477 (Evagr. H. E. iii. 4 ff.; Theod. Lect. i. 30 ff.; Theophan. Chron. pp. 104 ff.; Procop. B. V. i. 7, p. 195). At this period the relations between Zeno, Acacius, and Simplicius appear to have been amicable, if not cordial. They were agreed on the necessity of taking vigorous measures to affirm the decrees of the council of Chalcedon, and for a time acted in concert (Simplic. Epp. 5, 6). Before long a serious difference arose, when Acacias, in 479, consecrated a bishop of Antioch (Theophan. Chron. p. 110), and thus exceeded the proper limits of his jurisdiction. However, Simplicius admitted the appointment on the plea of necessity, while he protested against the precedent (Simplic. Epp. 14, 15). Three years later (482), on the death of the patriarch of Alexandria, the appointment of his successor gave occasion to a graver dispute. The Monophysites chose Petrus Mongus as patriarch, who had already been conspicuous among them; on the other side the Catholics put forward Johannes Talaia. Both aspirants lay open to grave objections. Mongus was, or at least had been, unorthodox; Talaia was bound by a solemn promise to the Emperor not to seek or (as it appears) accept the patriarchate (Liberat. c. 17; Evagr. H. E. iii. 12). Talaia at once sought and obtained the support of Simplicius, and slighted Acacius. Mongus represented to Acacius that he was able, if confirmed in his post, to heal the divisions by which the Alexandrine church was rent. Acacius and Zeno readily listened to the promises of Mongus, and in spite of the vehement opposition of Simplicius, received the envoys whom he sent to discuss the terms of reunion. Shortly afterwards the Henoticon (An Instrument of Union) was drawn up, in which the creed of Nicaea, as completed at Constantinople, was affirmed to be the one necessary and final definition of faith; and though an anathema was pronounced against Eutyches, no express judgment was pronounced upon the doctrine of the two Natures (Evagr. H. E. iii. 14) Mongus accepted the Henoticon, and was confirmed in his see. Talaia retired to Rome (482–483), and Simplicius wrote again to Acacius, charging him in the strongest language to check the progress of heresy elsewhere and at Alexandria (Simplic. Epp. 18, 19). The letters were without effect, and Simplicius died soon afterwards. His successor, Felix III. (II.), espoused the cause of Talaia with zeal, and despatched two bishops, Vitalis and Misenus, to Constantinople with letters to Zeno and Acacius, demanding that the latter should repair to Rome to answer the charges brought against him by Talaia (Felix, Epp. 1, 2). The mission utterly failed. Vitalis and Misenus were induced to communicate publicly with Acacius and the representatives of Mongus, and returned dishonoured to Italy (484). On their arrival at Rome a synod was held. They were themselves deposed and excommunicated; a new anathema was issued against Mongus, and Acacius was irrevocably excommunicated for his connexion with Mongus, for exceeding the limits of his jurisdiction, and for refusing to answer at Rome the accusations of Talaia (Evagr. H. E. iii. 21; Felix, Ephesians 6 ); but no direct heretical opinion was proved or urged against him. Felix communicated the sentence to Acacias, and at the same time wrote to Zeno, and to the church at Constantinople, charging every one, under pain of excommunication, to separate from the deposed patriarch (Epp. 9, 10, 12). Once again the envoy of the pope was seduced from his allegiance, and on his return to Rome fell under ecclesiastical censure (Felix, Ep. 11). For the rest, the threats of Felix produced no practical effect. The Eastern Christians, with very few exceptions, remained in communion with Acacias; Talaia acknowledged the hopelessness of his cause by accepting the bishopric of Nola; and Zeno and Acacius took active measures to obtain the general acceptance of the Henoticon. Under these circumstances the condemnation of Acacius, which had been made in the name of the Pope, was repeated in the name of the council of Chalcedon, and the schism was complete (485). Acacius took no heed of the sentence up to his death in 489, which was followed by that of Mongus in 490, and of Zeno in 491. Fravitas (Flavitas, Flavianus), his successor, during a very short patriarchate, entered on negotiations with Felix, which led to no result. The policy of Acacius broke down when he was no longer able to animate it. In the course of a few years all for which he had laboured was undone. The Henoticon failed to restore unity to the East, and in 519 the emperor Justin submitted to pope Hormisdas, and the condemnation of Acacius was recognized by the Constantinopolitan church.

Tillemont has given a detailed history of the whole controversy, up to the death of Fravitas, in his Mémoires, vol. xvi., but with a natural bias towards the Roman side. The original documents, exclusive of the histories of Evagrius, Theophanes, and Liberatus, are for the most part collected in the 58th volume of Migne's Patrologia. See also Hefele, Konz. Gesch. Bd. ii.