57 If there are many points which we treat scantily it is not because we have nothing to say, or do not know what has already been said; our purpose is, by abstaining from too laborious a process of argument, to render the results as attractive as possible to the reader. We know the deeds and words of our Lord, yet we know them not: we are not ignorant of them, yet they cannot be understood. The facts are real, but the power behind them is a mystery. We will prove this from His own words, For this reason does the Father love Me, because I lay down My life that I may take it up again.
No one takes it from Me, but I lay it down of Myself. I have power to lay it down and I have power to take it up again. This commandment received I from the Father. He lays down His life of Himself, but I ask who lays it down? We confess without hesitation, that Christ is God the Word: but on the other hand, we know that the Son of Man was composed of a soul and a body: compare the angel's words to Joseph, Arise and take the child and His mother, and go into the land of Israel; for they are dead who sought the soul of the child. Whose soul is it?
His body's, or God's? If His body's, what power has the body to lay down the soul, when it is only by the working of the soul that it is quickened into life? Again, how could the body, which apart from the soul is inert and dead, receive a command from the Father? But if, on the other hand, any man suppose that God the Word laid aside His soul, that He might take it up again, he must prove that God the Word died, that is, remained without life and feeling like a dead body, and took up His soul again to be quickened once more into life by it.
Source: On the Trinity (New Advent)