Ecclesiastical conflict meanwhile continues.

The Council of Chalcedon, convened in 451 by Pope Leo and the Eastern emperor, is the most notable event of Marcian's reign.

This fourth ecumenical council issues twenty-seven canons governing the Church; it upholds the orthodox Christian doctrine that Christ had two natures, divine and human, and rejects Monophysitism, which maintains that Christ had one divine nature.

As a result of this council, the Oriental Orthodox churches eventually become a separate communion.

More immediately, Jerusalem becomes a Patriarchate and Dioscorus of Alexandria is deposed as patriarch of Antioch.