There is no doubt therefore, dearly-beloved, that man's nature has been received by the Son of God into such a union that not only in that Man Who is the first-begotten of all creatures, but also in all His saints there is one and the self-same Christ, and as the Head cannot be separated from the members, so the members cannot be separated from the Head. For although it is not in this life, but in eternity that God is to be "all in all," yet even now He is the inseparable Inhabitant of His temple, which is the Church, according as He Himself promised, saying, "Lo! I am with you all the days till the end of the age."
From the opening of his most important work, the Tome of Leo:
Having read your letter, beloved, at the late arrival of which we are surprised , and having perused the detailed account of the bishops' acts , we have at last found out what the scandal was which had arisen among you against the purity of the Faith: and what before seemed concealed has now been unlocked and laid open to our view: from which it is shown that Eutyches, who used to seem worthy of all respect in virtue of his priestly office, is very unwary and exceedingly ignorant, so that it is even of him that the prophet has said: "he refused to understand so as to do well: he thought upon iniquity in his bed. " But what more iniquitous than to hold blasphemous opinions , and not to give way to those who are wiser and more learned than ourself. Now into this unwisdom fall they who, finding themselves hindered from knowing the truth by some obscurity, have recourse not to the prophets' utterances, not to the Apostles' letters, nor to the injunctions of the Gospel but to their own selves: and thus they stand out as masters of error because they were never disciples of truth. For what learning has he acquired about the pages of the New and Old Testament, who has not even grasped the rudiments of the Creed? And that which, throughout the world, is professed by the mouth of every one who is to be born again , is not yet taken in by the heart of this old man.