17 Behold, says some one, the Son has been sent; and the Father is greater, because He sent. Withdraw from the flesh; the old man suggests oldness in time. Let the ancient, the perpetual, the eternal, to you the new, call off your understanding from time to this. Is the Son less because He is said to have been sent? I hear of a sending, not a separation. But yet, says he, among men we see that he who sends is greater than he who is sent. Be it so; but human affairs deceive a man; divine things purge him.
Do not regard things human, in which the sender appears greater, the sent less; notwithstanding, things human themselves bear testimony against you. Just as, for example, if a man wishes to ask a woman to wife, and, not being able to do this in person, sends a friend to ask for him. And there are many cases in which the greater is chosen to be sent by the less. Why, then, would you now raise a captious objection, because the one has sent, the other is sent? The sun sends out a ray, but does not separate it; the moon sends out her sheen, but does not separate it; a lamp sheds light, but does not separate it: I see there a sending forth, not a separation.
For if you seek examples from human things, O heretical vanity, although, as I have said, even human things in some instances refute you, and convict of error; yet consider how different it is in the case of things human, from which you wish to deduce examples for things divine. A man that sends remains himself behind, while only the man that is sent goes forward. Does the man who sends go with him whom he sends? Yet the Father, who sent the Son, has not departed from the Son.
Hear the Lord Himself saying, “Behold, the hour is coming, when every one shall depart to his own, and you will leave me alone; but I am not alone, because the Father is with me.” How has He, with whom He came, sent Him? How has He, from whom He has not departed, sent Him? In another place He said, “The Father abiding in me does the works.” Behold, the Father is in Him, works in Him. The Father sending has not departed from the Son sent, because the sent and the sender are one.
Source: Tractates on the Gospel of John (New Advent)