The doctrinal differences that split the Church had to do with a heresy known as "Monophysitism." Monophysites believed that Christ had only one nature: divine. But orthodox belief held that Christ had two natures: both divine and human. This concept had been deliberately expressed at the Council of Chalcedon, an ecumenical council held in 451. Monophysites rejected this Council's decree concerning Christ's dual nature, and were therefore considered heretics by the Catholic Church.
The Monophysite heresy was very popular in Syria and Egypt, where anti-imperial feeling was strong, and it took hold in other parts of the Eastern Roman Empire (Byzantium), as well. However, the Patriarch of the church in the Byzantine capital of Constantinople, a leading figure of the Eastern Church, did not support Monophysitism. Up to 476 C.E., both Eastern and Western Churches were united against the Monophysite heresy.
In 475, Emperor Zeno was forced to flee Constantinople, and Basiliscus took over. Basiliscus favored Monophysitism, and clerics who followed the heresy saw some success while he was in power. Monophysitism took firm hold, particularly in Alexandria, Antioch and Jerusalem.
When Zeno returned to power 20 months later, he faced a severely divided empire. He sought a way to reunite the Monophysites with the Orthodox Church. For help he turned to Acacius, Patriarch of Constantinople.
Acacius had been a vocal supporter of Orthodoxy and had won the approval of Pope Simplicius. Now he turned his talents to drafting a letter of union. Known as the Henoticon, the letter endorsed the decrees of the councils of Nicaea and Constantinople, condemned the heretics Nestorius and Eutyches and their followers, and approved the anathemas of Cyril. However, it made no mention at all of the decrees of Chalcedon. This deliberate omission was, in itself, a concession to Monophysitism.
The Reception of the Henoticon:
Some Monophysite leaders, including Bishop Peter Mongus of Alexandria, accepted the Henoticon readily, but more extreme Monophysites rejected it outright. John Talaia, the orthodox bishop Acacius had deposed and replaced with Mongus, refused to sign it and sought help in Rome. The Western Church found the Henoticon completely unacceptable. Nevertheless, Zeno published the document in 482, deposing Catholic bishops and extreme Monophysites who refused to go along with the compromise.
Reaction of the Pope:
Pope Felix III despatched two bishops to Constantinople with letters to Zeno and Acacius, demanding that Acacius should go to Rome to answer charges brought against him by Talaia. Acacius managed to get the bishops to speak publicly with him, and they failed to induce the patriarch to go with them to Rome.
Upon their return to Rome, the bishops were condemned for their failure, and a synod was held in 484. At the synod, Felix deposed and excommunicated Acacius.
When word of his excommunication reached Acacius, he responded by striking Felix' name from his diptychs, and the schism began. Acacius continued as Patriarch until his death in 489.
Zeno died in 491, and at first his successor Anastasius I professed an adherence to Orthodoxy. But the representatives he sent to Pope Anastasius II were unable to reach an agreement, and the pope acquired an undeserved reputation as a heretic for dealing with the Monophysites at all.
As Emperor Anastasius grew older, he moved closer to the Monophysite doctrine and opposed the popes instead of trying to reconcile with them. During his reign, Byzantines supported the antipope Laurentius, which kept Pope Symmachus occupied for much of his pontificate. Symmachus' successor Hormisdas tried repeatedly to reconcile the churches, but his negotiations with Anastasius saw little progress.
Ending the Schism:
In 518 Emperor Anastasius died. His successor, Justin, was a staunch supporter of Orthodoxy, and he and Patriarch John of Cappadocia soon began fruitful negotiations with the pope. Hormisdas had authored a confession of faith, known as the Formula of Hormisdas, that the bishops in the East signed as part of the reconciliation. The names of Acacius, Anastasius and Zeno were stricken from the diptychs, and on March 28, 519, the reunion of the churches was ratified.
The Acacian Schism was the first significant break between the Eastern Orthodox Church and the Western Catholic Church. Although it only lasted 35 years, and although no significant doctrinal differences resulted, it was the first step down the road that led to the ultimate schism in 1054. While the two churches operated independently of one another, the Eastern Church developed a strong sense of identity, and the papacy established itself more firmly as the head of the Western Church.