I was just reading this in today's Synaxarion:
The same day (Sept. 25), Commemoration of the Great Earthquake at Constantinople AD 447, and of the Miracle of the TRISAGION
In the reign of the Emperor Theodosius II (408-50) God, bountiful and rich in mercy, caused the ground to shake dreadfully for almost four months to warn Christian people to be always ready for the Great Day when the dead shall be raised, heaven and earth shall be changed into a new form, and every man shall be summoned to judgment upon the uprightness of his faith and the purity of his life. Fear gripped the entire City. The Emperor, the Patriarch Saint Proclus (20 Nov.) and all the people of Constantinople, went in procession barefoot to the parade ground of the Hebdomon, where they made earnest prayer to God for their safety. About the third hour, the ground once more began shaking, and a young boy was suddenly taken up into the skies by the strength of the Almighty, while the terrified people cried: Kyrie eleison with redoubled fervor. When he came down from on high, the child declared he had been taken up amid the choirs of angels, who were singing: Holy God, Holy Mighty, Holy Immortal, have mercy upon us! and that a voice had commanded him to tell the Patriarch that the people ought to make their supplications to God in this way, without adding anything. The Patriarch instructed the chanters and people to intone this hymn, that joins confession of the three Divine Persons, to the cry of the Seraphim in the vision of Isaiah (Isa. 6:3). Whereupon the earth stopped shaking and the child gave up his soul to God.
The most pious Empress Pulcheria (10 Sept.) enjoined Saint Proclus to order this hymn to be solemnly chanted henceforth in the Liturgy. At the holy Council of Chalcedon (451), the Fathers from the Roman diocese of Asia greeted the proclamation of the true Faith by chanting the Trisagion, which has since become an essential element in the private as well as in the common prayer of Orthodox Christians.
When Peter the Fuller, the usurping Patriarch of Antioch, wanted to spread the poison of the theopaschite heresy (a type of Monophysism), he had the expression: who was crucified for us, added to the Thrice-holy hymn (c. 468). The Orthodox remembered that when the child was miraculously taken into heaven, the divine voice had distinctly forbidden any addition to the hymn, and they strove hard to maintain the Trinitarian understanding of the Trisagion against a Christological interpretation. So by right glorification of the one only God in three Persons the Orthodox Faith was kept.
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