The council of 451 was called after a gathering at Ephesus two years earlier, which its opponents named the robber synod, had vindicated the teaching that Christ had one nature after the union. Chalcedon reversed that decision. Its Definition states that Christ is acknowledged in two natures, without confusion, without change, without division, without separation, in one person and one hypostasis, and it endorsed the letter of Pope Leo that argued the same case. The formula was built to exclude two positions at once, the merging of the divine and human into one nature and their separation into two persons, and its four negatives are the most quoted words the early councils produced. It was not received everywhere. The Armenian, Coptic, Syriac and Ethiopian churches rejected it and remain outside communion with the Chalcedonian churches, which makes this the oldest division in Christianity still visibly in force.
Modern dialogue has produced joint statements in which both sides say they profess the same faith in Christ in different formulations, without restoring communion. The council also passed a canon granting Constantinople rank equal to Rome in honour on the ground that it was the imperial city, which Leo refused to accept and which became a standing grievance between east and west long before the schism of 1054.