II. He commends Leo's zeal against the Manichees, and latterly against Eutychianism, as evidenced especially in the Tome.
Of which thing indeed, though there are many other proofs to be found, your zeal against the ill-famed Manichæans is proof enough, that zeal which your holiness has of late years displayed, thereby revealing the intensity of your devotion to God in things Divine. Proof enough, too, of your Apostolic character is what you have now written. For we have met with what your holiness has written about the Incarnation of our God and Saviour, and have admired the careful diligence of the work. For it has proved both points equally well, viz., the Eternal Godhead of the Only-begotten of the Eternal Father, and at the same time His manhood of the seed of Abraham and David, and His assumption of a nature in all things like ours, except in this one thing, that He remained free from all sin: for sin is engendered not of nature, but of free will. This also was contained in your letter, that the only-begotten Son of God is One and His Godhead impassible, irreversible, unchangeable even as the Father who begot Him and the All-holy Spirit.
And since the Divine nature could not suffer, He took the nature that could suffer to this end, that by the suffering of His own Flesh He might give exemption from suffering to those that believed on Him. These points, and all that is akin thereto, the letter contained. And we, admiring your spiritual wisdom, extolled the grace of the Holy Ghost which spoke through you, and ask and pray, and beg and beseech your holiness to come to the rescue of the churches of God that are now tempest tossed.
Source: Letters (New Advent)