Miaphysitism

Cyril of Alexandria, defending the unity of Christ against Nestorius earlier in the fifth century, taught that after the incarnation the Word of God had one nature, mia physis, made flesh, a phrase he believed he had inherited from earlier fathers though it seems to have originated as a mistaken attribution. When the Council of Chalcedon met in 451, it defined Christ instead as one person known in two natures, divine and human, united without confusion, change, division or separation, a formula meant to protect both Christ's full divinity and full humanity against different errors on either side.

The bishops of Egypt, Syria and Armenia present at or reacting to Chalcedon read the two-natures language as a betrayal of Cyril's formula and, by extension, as tipping back toward the very error, dividing Christ into two separate subjects, that Cyril had spent his career opposing. Alexandria's own patriarch, Dioscorus, was deposed at the council, and the churches that followed his Christology refused to accept Chalcedon's definition, a rupture that has lasted, largely unhealed institutionally, for more than fifteen centuries.

Each of the resulting Oriental Orthodox churches, Coptic, Syriac, Armenian, Ethiopian, Eritrean and Malankara, developed its own liturgy, calendar and patriarchate independently, but all trace their distinct existence to this single fifth-century Christological disagreement rather than to any difference in geography or governance alone.

For most of their separate history, the Chalcedonian churches called their Oriental Orthodox counterparts monophysite, a term that in Greek suggests Christ has only a single, effectively divine nature with his humanity absorbed or dissolved into it, the actual heresy of Eutyches that both Chalcedon and Cyril himself had opposed. The Oriental Orthodox churches have long rejected the label as a caricature, insisting their own position, better rendered miaphysite, one united nature rather than a single simple nature, never denied Christ's full and complete humanity, only refused to speak of it as a separate nature alongside his divinity.

Sustained modern ecumenical dialogue, particularly a series of unofficial and later official conversations between Eastern Orthodox and Oriental Orthodox theologians from the 1960s onward, has concluded that the two families largely mean the same thing by different technical vocabulary, Chalcedonian two-natures language on one side and Cyrilline one-nature language on the other, both intending to affirm Christ as fully God and fully human in a single, undivided person. Several joint statements have declared the underlying faith to be substantially shared.

Formal reunion has not followed the theological agreement, since centuries of separate liturgical, canonical and jurisdictional development stand alongside the Christological question, but the modern consensus has meaningfully changed how each side now describes the other, from heretic to estranged sibling.