St Cyril of Alexandria, Bishop, Confessor and Doctor.
Jesus must be a Man to atone for man's sin and a God to make reparation for the offence against God. The Cycle celebrates on this day the feast of the Doctor who raised his voice in the Church (Introit) to condemn in the name of Pope Celestine, at the Council of Ephesus (431), the heresy of Nestorius. Crushing under foot the salt which has lost its savour (Gospel) St. Cyril, bishop of Alexandria, attacks the heresiarch who instead "of the true doctrine only teaches fables" (Epistle). He affirms that in Jesus there is only one divine Person and that consequently the Saviour is at the same time God and Man and that His Mother is the Mother of God (Collect). He died in 444.
In medio Ecclesiae aperuit os ejus: et implevit eum Dominus spiritu sapientiae et intellectus: stolam gloriae induit eum. * Bonum est confiteri Domino: et psallere nomini tuo, Altissime.
In the midst of the Church the Lord opened his mouth: and He filled him with the spirit of wisdom and understanding: He clothed him with a robe of glory. * It is good to give praise to the Lord: and to sing to Thy name, O most High.
(Ecclesiasticus 15:5 and Psalm 91:2 from the Introit of Mass)
Deus, qui beatum Cyrillum Confessorem tuum atque Pontificem divinae maternitatis beatissimae Virginis Mariae assertorem invictum effecisti: concede, ipso intercedente, ut; qui vere eam Genitricem Dei credimus, materna ejusdem protectione salvemur.
O God, who didst render blessed Cyril, Thy confessor and bishop, the invincible champion of the divine motherhood of the most blessed Virgin Mary; grant by his intercession, that we who believe her to be truly the Mother of God, may be saved through her maternal intercession.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/04592b.htm