Ephesus had confessed the unity of Christ's person. But a new error pressed from the opposite direction: Eutyches, an archimandrite of Constantinople, was understood to teach that Christ's humanity was absorbed into his divinity, leaving a single, blended nature. If Ephesus guarded against dividing Christ, Chalcedon was called to guard against confusing him.
The Definition
Drawing on Cyril of Alexandria and on the Tome of Leo, bishop of Rome, the council defined that Christ is acknowledged in two natures — divine and human — 'without confusion, without change, without division, without separation.' The two natures are not blended into a third thing, nor are they split into two persons. The distinction of the natures is preserved; their union in one person is confessed.
A Council Not Universally Received
Chalcedon is received as ecumenical by the Eastern Orthodox, Roman Catholic, and most Protestant traditions. The Oriental Orthodox churches — Coptic, Armenian, Syriac, Ethiopian, and others — did not receive it, holding that its language risked the very division it sought to prevent. That separation, ancient and still significant, is described on the Oriental Orthodox tradition pages. Here the council is recorded for the confession it framed.