11 Let us turn, then, to what is true and easily understood. Take the case of any man, who does not consist of the word and soul and flesh, but only of soul and flesh; and let us inquire how any such man lays down his life. Can no ordinary man do so? You may say to me: No man has power to lay down his life [soul], and to take it again. But were not a man able to lay down his life, the Apostle John would not say, “As Christ laid down his life for us, even so ought we also to lay down our lives for the brethren.” Therefore may we also (if only we are filled with His courage, for without Him we can do nothing) lay down our lives for the brethren.
When some holy martyr has laid down his life for the brethren, who laid it down, and what laid he down? If we understand this, we shall perceive in what sense it was said by Christ, “I have power to lay down my life.” Are you prepared, O man, to die for Christ? I am prepared, he replies. Let me repeat the question in other words. Are you prepared to lay down your life for Christ? And to these words he makes me the same reply, I am prepared, as he had, when I said, Are you prepared to die?
To lay down one's life [soul], is, then, the same as to die. But in whose behalf is the sacrifice in this case? For all men, when they die, lay down their life; but it is not all who lay it down for Christ. And no one has power to resume what he has laid down. But Christ both laid it down for us, and did so when it pleased Him; and when it pleased Him, He took it again. To lay down one's soul then, is to die. As also the Apostle Peter said to the Lord: “I will lay down my life [soul] for Your sake;” that is, I will die for Your sake.
View it, then, as referable to the flesh: the flesh lays down its life, and the flesh takes it again; not, indeed, the flesh by its own power, but by the power of Him that inhabites it. The flesh, then, lays down its life in expiring. Look at the Lord Himself on the cross: He said, “I thirst:” those who were present dipped a sponge in vinegar, fastened it to a reed, and applied it to His mouth; then, having received it, He said, “It is finished;” meaning, All is fulfilled which had been prophesied regarding me as, prior to my death, still in the future.
And because He had the power, when He pleased, to lay down His life, after He had said, “It is finished,” what adds the evangelist? “And He bowed His head, and gave up the spirit.” This is to lay down the soul [life]. Only let your Charity attend to this. “He bowed His head, and gave up the spirit.” Who gave up what gave He up? He gave up the spirit; His flesh gave it up. What means, the flesh gave it up? The flesh sent it forth, breathed it out. For so, in becoming separated from the spirit, we are said to expire.
Just as getting outside the paternal soil is to be expatriated, turning aside from the track is to deviate; so to become separated from the spirit is to expire; and that spirit is the soul [life]. Accordingly, when the soul quits the flesh, and the flesh remains without the soul, then is a man said to lay down his soul [his human life]. When did Christ lay down His life? When it pleased the Word. For sovereign authority resided in the Word; and therein lay the power to determine when the flesh should lay down its life, and when it should take it again.
Source: Tractates on the Gospel of John (New Advent)