Theodosius II, Roman emperor of the East, convened a Church council in Ephesus on August 8 A.D. 449, the year before his fatal fall from a horse on July 28, 450. This confusing council is known by its opponents as the Robber Council. The following is my attempt to make sense of it. Should you want to clarify further or correct errors, please do so.
Theologians and bishops were still discussing the tenets of the Christian faith throughout the 4th and 5th centuries. Although doctrine wasn’t settled, deviations were treated seriously.
The Second Council of Ephesus (the other name of the Robber Council) was convened to deal with the issue of the nature of Christ. Theodosius II supported adherents of what is now considered the Monophysite “heresy”. Eutyches (c. 375-454), a monophysite authority figure in the eastern monastic community (called an Archimandrite), opposed the Nestorians, not because Nestorians believed there was only a divine nature to Christ, as the monophysites did, but because the Nestorians rejected the Virgin Mary as too human to be the mother of God. When Eutyches and the eastern monophysites condemned the Nestorians, Bishop Eusebius of Dorylaeum in Asia Minor declared the monophysite belief heretical.
The Nestorians weren’t considered orthodox, either. The Roman Pope Celestine I was unhappy with Nestorius, who had been named patriarch of Constantinople, in 428. He commissioned Cyril (d. 444), Patriarch of Alexandria, to proceed against Nestorius at the Third Ecumenical Council, in 431. When Pope Celestine I excommunicated Nestorius, the eastern bishops excommunicated Patriarch Cyril. (Incidentally, this was also the council that condemned Pelagianism.) The eastern bishops started their own competing council led by John of Antioch. (So we have Rome, Alexandria, Antioch, and Constantinople all with their own grudges and beliefs.) Theodosius II had tried to remain neutral, but at this point, he took sides — with the anti-Nestorian Cyril. Eutyches, an anti-Nestorian monophysite, also opposed the Antiochenes.
Just because Eutyches had been trained in the theology popular in Alexandria, and just because Alexandria and Rome had led a tag-team attack on Nestorius, did not mean the Roman pope approved Eutyches’ beliefs. Nor did the Orthodox (vs. Antiochene) patriarch. When Eutyches was called to the presence of the new (Orthodox) patriarch of Constantinople, Flavian, he insisted his beliefs were in accord with the Church Fathers from the Council of Nicaea. He explained his belief as that Christ had two natures prior to incarnation and but one afterwards.
Eutyches was excommunicated. Pope Leo I also condemned Eutychianism. Not all the Church bigwigs did, though. In Alexandria, Patriarch Dioscorus supported Eutyches and persuaded Emperor Theodosius II to convene a Church council. Theodosius did, with the result that Eutyches was reinstated. However, this council is called the Robber Council because it was not considered a proper council by those conveniently excluded from participation and is recognized by neither the Roman nor Orthodox Churches.
Since Theodosius died shortly thereafter, his successor, Marcian (396-457) was able to convene a new council to obviate the work of the Robber Council, but the Robber Council was a definite step in the rift between the church of the East and West.