37 The very recalling of the memory of these distinguished and glorious events in the history of the Church naturally leads us to turn our thoughts to the Orientals with a yet more loving warmth of paternal affection. For the oecumenical council of Chalcedon is a monument of their outstanding glory, and one which, without doubt, will live throughout the ages. For in this council under the leadership of the Apostolic See, an assembly of 600 Oriental bishops vigilantly defended and wonderfully expounded against the rashness of the innovator, the doctrine of the unity of Christ, in whose person meet without confusion two distinct natures, the divine and the human. But alas! for long centuries many of those who dwell in the East have unhappily fallen away from the unity of the Mystical Body of Christ, of which the hypostatic union is the most luminous prototype. Would it not be holy, salutary and in accordance with the will of God that at last all these should return to the one sheepfold of Christ?
Source: Sempiternus Rex Christus (Vatican.va)