As Doctor of the Church and defender of civilisation, he gained signal victories over heretics and barbarians.
He opposed Eutyches who denied the two natures of Christ, and Nestorius who affirmed that there were two persons in Jesus; which fatal doctrines ended towards the destruction of the great work of Redemption celebrated by the Paschal Cycle. The Council of Chalcedon exclaimed: "Peter has spoken by the mouth of Leo." Wherefore the Gospel reminds us of the prerogatives of the chief Pastor of the Church.
St. Leo stopped the advance of Attila, "the scourge of God", at the gates of Rome and decided him to retire to Pannonia. He died on April 11th, 461.
Let us ask God, through his intercession, to apply to us the merits of the risen Christ, that we may be freed from our sins (Collect).
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)
Exaudi, quaesumus, Domine, preces nostras, quas in beati Leonis Confessoris tui atque Pontificis solemnitate deferimus: et, qui tibi digne meruit famulari, ejus intercedentibus meritis ab omnibus nos absolve peccatis.
Graciously hear our prayers, O Lord, we beseech Thee, which we offer in the solemn commemoration of blessed Leo, Thy confessor and bishop; and absolve us from all sins by the interceding merits of him, who served Thee worthily.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/09154b.htm