Orientalis Ecclesiae §4

⁴ Born of distinguished family, he was raised to the See of Alexandria - so tradition tells us - in the year 412. His first conflict was with the Novatians and others who attacked the integrity and purity of the faith, and against these he preached, wrote, and issued decrees, ever alert, ever fearless. Later, when the blasphemous heresy of Nestorius began to spread gradually through the East the watchful Pastor was quick to perceive the growth of these new errors and zealous in protecting his flock against them. Throughout this stormy period, and especially at the Council of Ephesus, he showed himself the invincible champion and learned teacher of the divine maternity of the Virgin Mary, of the hypostatic union in Christ, and of the Primacy of the Roman Pontiff. But the leading part which St. Cyril played in these important events has already been admirably described and explained by Our immediate Predecessor of happy memory Pius XI, in the Encyclical Lux Veritatis with which in the year 1931 he celebrated the fifteenth centenary of that ecumenical Council, and therefore it would be superfluous to enter into the details of it here.