2 As our predecessor Pope Pius XI of happy memory solemnly commemorated the Nicene council in 1925 in the sacred city, and by his encyclical letter Lux Veritatis recalled the sacred council of Ephesus in 1931, so we by the present letter pay a tribute of equal honor to the council of Chalcedon. For inasmuch as both councils, Ephesus and Chalcedon, were concerned with the hypostatic union of the Incarnate Word, they are intimately connected with one another. From the earliest times both councils have enjoyed the highest honor, equally in the East, where they are celebrated in the liturgy, and in the West. St. Gregory the Great bears witness in the West to this fact when he praises both councils together with two of the preceding century, namely, those of Nicea and Constantinople, in the memorable sentence: - 'On them, as a four-cornered stone, the building of the holy faith stands erect, and whoever does not hold their firm doctrine, whatever may be his life or activity, even if he seems to be a rock, nevertheless lies outside the building' ( Regist. Epist. i, 25 . Pl . lxxvii, 478, ed. Ewald i, 36).
Source: Sempiternus Rex Christus (Vatican.va)