9 Whenever, then, the Father shows to Christ's members, He shows to Christ. A certain great but yet real miracle happens. There is a showing to Christ of what Christ knew, and it is shown to Christ through Christ. A marvelous and great thing it is, but the Scripture so says. Shall we contradict the divine declarations? Shall we not rather understand them, and of His own gift render thanks to Him who freely bestowed it on us? What is this that I said, “is shown to Christ through Christ”?
Is shown to the members through the head. Lo, look at this in yourself. Suppose that with your eyes shut you would take up something, your hand knows not whither to go; and yet your hand is at any rate your member, for it is not separated from your body. Open your eyes, now the hand sees whither it may go; while the head showed, the member followed. If, then, there could be found in yourself something such, that your body showed to your body, and that through your body something was shown to your body, then do not marvel that it is said there is shown to Christ through Christ.
For the head shows that the members may see, and the head teaches that the members may learn; nevertheless one man, head and members. He willed not to separate Himself, but deigned to attach Himself to us. Far was He from us, yea, very far. What so far apart as the creature and the Creator? What so far apart as God and man? What so far as justice and iniquity? What so far as eternity and mortality? Behold, so far from us was the Word in the beginning, God with God, by whom all things were made. How, then, was He made near, that He might be what we are, and we in Him? “The Word was made flesh, and dwelt in (among) us.”
Source: Tractates on the Gospel of John (New Advent)