Very often it is not entire bodies that are buried, but only very small remains, yet nevertheless we speak of the body by the name that belongs to the whole man. It was in this sense that the angel called the body of the Lord, “Lord,” because it was the body of the Lord of the universe. Moreover the Lord Himself promised to give on behalf of the life of the world, not His invisible nature, but His body. “For,” He says, “the bread that I will give is my flesh which I will give for the life of the world,” and when He took the symbol of divine mysteries, He said, “This is my body which is given for you.” Or according to the version of the Apostle, “broken.” In no place where He spoke of the passion did He mention the impassible Godhead.
It is therefore before all things necessary that the question should be put to those who are endeavouring to contradict us whether they confess that the perfect manhood was assumed by God the Word, and assert the union to have been made without confusion. Once let these points be admitted, and the rest will follow in due course, and the passion will be attributed to the passible nature. I have now summed up these heads and have exceeded the limits of my letter. I have sent also what I lately wrote at the suggestion of a very godly and holy man of God, the lord— in the form of a concise instruction designed to teach the truth of the apostolic doctrines. Should I find a good copyist, I will also send your holiness what I have written in the form of a dialogue, extending the argument, and strengthening my positions, by the teaching of the Fathers. I have moreover now sent a few statements of the ancient teachers, sufficient to show the drift of their instruction. Give me in return, most godly sir, the succour of your prayers, that I may pass through the terrible tempest and reach the quiet haven of the Saviour.
Source: Letters (New Advent)