The Son and Holy Spirit are Not Therefore Less Because Sent. The Son is Sent Also by Himself. Of the Sending of the Holy Spirit
7 But being proved wrong so far, men betake themselves to saying, that he who sends is greater than he who is sent: therefore the Father is greater than the Son, because the Son continually speaks of Himself as being sent by the Father; and the Father is also greater than the Holy Spirit, because Jesus has said of the Spirit, “Whom the Father will send in my name;” and the Holy Spirit is less than both, because both the Father sends Him, as we have said, and the Son, when He says, “But if I depart, I will send Him unto you.” I first ask, then, in this inquiry, whence and whither the Son was sent. “I,” He says, “came forth from the Father, and have come into the world.” Therefore, to be sent, is to come forth forth from the Father, and to come into the world. What, then, is that which the same evangelist says concerning Him, “He was in the world, and the world was made by Him, and the world knew Him not;” and then he adds, “He came unto His own?” Certainly He was sent there, whither He came; but if He was sent into the world, because He came forth from the Father, then He both came into the world and was in the world. He was sent therefore there, where He already was. For consider that, too, which is written in the prophet, that God said, “Do not I fill heaven and earth?” If this is said of the Son (for some will have it understood that the Son Himself spoke either by the prophets or in the prophets), whither was He sent except to the place where He already was? For He who says, “I fill heaven and earth,” was everywhere. But if it is said of the Father, where could He be without His own word and without His own wisdom, which “reaches from one end to another mightily, and sweetly orders all things?” But He cannot be anywhere without His own Spirit. Therefore, if God is everywhere, His Spirit also is everywhere. Therefore, the Holy Spirit, too, was sent there, where He already was. For he, too, who finds no place to which he might go from the presence of God, and who says, “If I ascend up into heaven, You are there; if I shall go down into hell, behold, You are there;” wishing it to be understood that God is present everywhere, named in the previous verse His Spirit; for He says, Whither shall I go from Your Spirit? Or whither shall I flee from Your presence?
8. For this reason, then, if both the Son and the Holy Spirit are sent there where they were, we must inquire, how that sending, whether of the Son or of the Holy Spirit, is to be understood; for of the Father alone, we nowhere read that He is sent. Now, of the Son, the apostle writes thus: “But when the fullness of the time had come, God sent forth His Son, made of a woman, made under the law, to redeem them that were under the law.” “He sent,” he says, “His Son, made of a woman.” And by this term, woman, what Catholic does not know that he did not wish to signify the privation of virginity; but, according to a Hebraism, the difference of sex? When, therefore, he says, “God sent His Son, made of a woman,” he sufficiently shows that the Son was “sent” in this very way, in that He was “made of a woman.” Therefore, in that He was born of God, He was in the world; but in that He was born of Mary, He was sent and came into the world. Moreover, He could not be sent by the Father without the Holy Spirit, not only because the Father, when He sent Him, that is, when He made Him of a woman, is certainly understood not to have so made Him without His own Spirit; but also because it is most plainly and expressly said in the Gospel in answer to the Virgin Mary, when she asked of the angel, “How shall this be?” “The Holy Ghost shall come upon you, and the power of the Highest shall overshadow you.” And Matthew says, “She was found with child of the Holy Ghost.” Although, too, in the prophet Isaiah, Christ Himself is understood to say of His own future advent, “And now the Lord God and His Spirit has sent me.”
Source: On the Holy Trinity (New Advent)