I. He congratulates the Emperor on his share in the triumph of the Catholic Faith.
Leo, the Bishop, to Marcian Augustus.
By the great bounty of God's mercy the joys of the whole Catholic Church were multiplied when through your clemency's holy and glorious zeal the most pestilential error was abolished among us; so that our labours the more speedily reached their desired end, because your God-serving Majesty had so faithfully and powerfully assisted them. For although the liberty of the Gospel had to be defended against certain dissentients in the power of the Holy Ghost, and through the instrumentality of the Apostolic See, yet God's grace has shown itself more manifestly (than we could have hoped) by vouchsafing to the world that in the victory of the Truth only the authors of the violation of the Faith should perish and the Church restored to her soundness. Accordingly the war which the enemy of our peace had stirred up, was so happily ended, the Lord's right hand fighting for us, that when Christ triumphed all His priests shared in the one victory, and when the light of Truth shone forth, only the shades of error, with its champions, were dispelled. For as in believing the Lord's own resurrection, with a view to strengthen the beginnings of Faith, confidence was much increased by the fact that certain Apostles doubted of the bodily reality of our Lord Jesus Christ, and by examining the prints of the nails and the wound of the spear with sight and touch removed the doubts of all by doubting; so now, too, while the misbelief of some is refuted, the hearts of all hesitaters are strengthened, and that which caused blindness to some few avails for the enlightenment of the whole body. In which work your clemency duly and rightly rejoices, having faithfully and properly provided that the devil's snares should do no hurt to the Eastern churches, but that to propitiate God everywhere more acceptable holocausts should be offered; seeing that through the mediator between God and man, the Man Christ Jesus, one and the self-same creed is held by people, priests, and princes, O most glorious son and most clement Augustus.
Source: Letters (New Advent)