St. Cyril, Patriarch of Alexandria

"St. Cyril, Patriarch of Alexandria, one of the metropolitan sees of the Christian Church in the east, was one of the great defenders of the faith against the heresy of Nestorius who denied the oneness of person in Jesus Christ. At the Council of Ephesus in 431, over which he presided in the Pope's name, and at his instigation, it was defined that Christ, the Son of God, is at the same time God and man, and the Blessed Virgin Mary, His mother, is truly the Mother of God. St. Cyril died in 444. The Church venerates him as one of her great doctors. His commentary on the Gospel of St. John is one of the richest doctrinally of those left us by the Fathers of the Church.

According to the 1962 Missal of Bl. John XXIII the Extraordinary Form of the Roman Rite, the feast of St. Cyril of Alexandria is celebrated on February 9." (from Catholic Culture)

You can read Orientalis Ecclesiae (On St. Cyril, Patriarch Of Alexandria)
by Pope Pius XII here:

To Our Venerable Brethren the Patriarchs, Primates, Archbishops, Bishops and other Ordinaries at Peace and in Communion with the Apostolic See

Venerable Brethren, Health and Our Apostolic Benediction

St. Cyril, Patriarch of Alexandria, glory of the Eastern Church and celebrated champion of the Virgin Mother of God, has always been held by the Church in the highest esteem, and We welcome the opportunity of recalling his merits in this brief Letter, now that fifteen centuries have passed since he happily exchanged this earthly exile for his heavenly home.

2. Our Predecessor St. Celestine I hailed him as 'good defender of the Catholic faith,'[1] as 'excellent priest,'[2] as 'apostolic man.'[3] The ecumenical Council of Chalcedon not only used his doctrine for the detecting and refuting of the latest errors, but went so far as to compare it with the learning of St. Leo the Great;[4] and in fact the latter praised and commended the writings of this great Doctor because of their perfect agreement with the faith of the holy Fathers.[5] The fifth ecumenical Council, held at Constantinople, treated St. Cyril's authority with similar reverence[6] and many years later, during the controversy about the two wills in Christ, his teaching was rightly and triumphantly vindicated, both in the first Lateran Council[7] and in the sixth ecumenical Council, against the false charge of being tainted with the error of Monothelitism. He was, as Our saintly Predecessor Agatho proclaimed, 'a defender of the truth'[8] and 'a consistent teacher of the orthodox faith.'[9]

3. We therefore think it proper in this Letter to give some account of his spotless life, faith, and virtue; and this for the benefit of all, but especially of those who belong to the Eastern Church and therefore have good reason to be proud of this luminary of Christian wisdom, this valiant hero of the apostolate.

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[10] 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.

5. For Cyril, however, it was not enough to fight vigorously against heresies as they arose, not enough to guard the integrity of Catholic doctrine with energy and solicitude and throw the fullest possible light upon it; he was also untiring in his labors to recall his erring brethren to the straight path of the truth. For when the Bishops of the Province of Antioch were still refusing to recognize the authority of the holy Council of Ephesus, it was due to his efforts that they were at length, after long vicissitudes, brought to complete agreement. And it was only after he had succeeded with God's help in accomplishing this happy reunion and in guarding and securing it against misconceptions that, being now ripe for the reward of everlasting glory, he was taken up to heaven in the year 444, mourned by all men of good will... (Continued here.)