In the year 1931, amid the applause of the whole Catholic world, solemn rites were celebrated to mark the completion of the fifteen centuries which had elapsed since the Council of Ephesus, moving against the Nestorian heresy, had acclaimed the Blessed Virgin Mary, of whom Jesus was born, as Mother of God. This acclamation had been made by the Fathers of the Church under the leadership of Pope Celestine. Pius XI, as Supreme Pontiff, wished to commemorate the notable event and to give lasting proof of his devotion to Mary.
Homilia 1. de Laud. Virg. Matris