The Council of Ephesus was followed in the Byzantine church itself, and in Alexandria in particular, by the development of new movements in opposition to Nestorianism. The followers of Cyril of Alexandria, while they believed in the preponderance of the divine nature over the human in Jesus Christ, arrived at the conclusion that the human was completely absorbed by the divine substance; hence Jesus Christ possessed but one - divine - nature. This new teaching was called Monophysitism, or the Monophysitic doctrine, and its followers are known as the Monophysites (from the Greek μόνος, one [single], and φύσις, nature). Monophysitism made great progress with the aid of two ardent Monophysites, the Alexandrian bishop Dioscorus, and Eutyches, the archimandrite of a monastery in Constantinople. The Emperor sided with Dioscorus, whom he considered an advocate of the ideas of Cyril of Alexandria.
The new teaching was opposed by the patriarch of Constantinople and by Pope Leo I the Great. Dioscorus then urged the Emperor to call a council in the year 449 at Ephesus, which is known as the Robber Council. The Alexandrian party of Monophysites headed by Dioscorus, who presided at the council, forced members of the council who did not agree with them to recognize the teaching of Eutyches (Monophysitism) as orthodox and to condemn the opponents of the new doctrine. The Emperor ratified the decisions of the council, officially recognizing it as an ecumenical council. Naturally the council failed to establish harmony in the church. A period of stormy disturbances followed, during which Theodosius died, leaving to his successors the solution of the problem of Monophysitism, highly important in Byzantine history.
Besides the stormy and significant religious events of the period of Theodosius there were a number of events in the internal life of the Empire which marked this epoch as historically important.
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Reference address : https://www.ellopos.net/elpenor/vasilief/third-ecumenical-council.asp?pg=3