He confirms what has been already said, by the parable of the rich man who went into a far country to receive for himself a kingdom; and shows that when the Son delivers up the kingdom to the Father, we must not regard the fact that the Father is said to put all things in subjection under Him, in a disparaging way. Here we are the kingdom of Christ, and in Christ's kingdom. Hereafter we shall be in the kingdom of God, where the Trinity will reign together.
145. In divine fashion has He represented that parable of the rich man, who went to a far-off country to receive a kingdom, and to return, thus describing Himself in the substance of the Godhead, and of His Manhood. For He being rich in the fullness of His Godhead, Who was made poor for us though He was rich and an eternal King, and the Son of an eternal King; He, I say, went to a foreign country in taking on Him a body, for He entered upon the ways of men as though upon a strange journey, and came into this world to prepare for Himself a kingdom from among us.
146. Jesus therefore came to this earth to receive for Himself a kingdom from us, to whom He says: “The kingdom of God is within you.” This is the kingdom which Christ has received, this the kingdom which He has delivered to the Father. For how did He receive for Himself a kingdom, Who was a King eternal? “The Son of Man therefore came to receive a kingdom and to return.” The Jews were unwilling to acknowledge Him, of whom He says: “They which would not that I should reign over them, bring hither and slay them.”
147. Let us follow the course of the Scriptures. He Who came will deliver up the kingdom to God the Father; and when He has delivered up the kingdom, then also shall He be subject to Him, Who has put all things in subjection under Him, that God may be all in all. If the Son of God has received the kingdom as Son of Man, surely as Son of Man also He will deliver up what He has received. If He delivers it up as Son of Man, as Son of Man He confesses His subjection indeed under the conditions of the flesh, and not in the majesty of His Godhead.
148. And do you make objections and contemn Him, because God has put all things in subjection under Him, when you hear that the Son of Man delivers up the kingdom to God, and hast read, as we said in our earlier books: “No man can come to Me, except the Father draw him; and I will raise him up at the last day”? If we follow it literally, see rather and notice the unity of honour each gives to other: The Father has put all things in subjection under the Son, and the Son delivers the kingdom to the Father. Say now which is the greater, to deliver up, or to raise up to life? Do we not after human fashion speak of the service of delivering up, and the power of raising to life? But both the Son delivers up to the Father, and also the Father to the Son. The Son raises to life, and the Father also raises to life. Let them create the fiction of a blasphemous division where there is a unity of power.
149. Let the Son then deliver up His kingdom to the Father. The kingdom which He delivers up is not lost to Christ, but grows. We are the kingdom, for it was said to us: “The kingdom of God is within you.” And we are the kingdom, first of Christ, then of the Father; as it is written: “No man comes to the Father, but by Me.” When I am on the way, I am Christ's; when I have passed through, I am the Father's; but everywhere through Christ, and everywhere under Him.
150. It is a good thing to be in the kingdom of Christ, so that Christ may be with us; as He Himself says: “Lo I am with you always, even unto the end of the world.” But it is better to be with Christ: “For to depart and be with Christ is far better.” Though we are under sin in this world, Christ is with us, that “by the obedience of one man many may be made just.” And if I escape the sin of this world, I shall begin to be with Christ. And so He says: “I will come again, and receive you unto Myself;” and further on: “I will that where I am, there you may be also with Me.”
151. Therefore we are now under Christ's rule, while we are in the body, and are not yet stripped of the form of a servant, which He put upon Him, when He “emptied Himself.” But when we shall see His glory, which He had before the world was, we shall be in the kingdom of God, in which are the patriarchs and prophets, of whom it is written: “When you shall see Abraham, Isaac, and Jacob, and all the prophets in the kingdom of God;” and shall thus acquire a deeper knowledge of God.
152. But in the kingdom of the Son the Father also reigns; and in the kingdom of the Father the Son also reigns: for the Father is in the Son, and the Son in the Father; and in whomsoever the Son dwells, in him also the Father dwells; and in whomsoever the Father dwells, in him also the Son dwells, as it is written: “Both I and My Father will come to Him, and make Our abode with Him.” Thus as there is one dwelling, so also there is one kingdom. Yea, and so far is the kingdom of the Father and of the Son but one, that the Father receives what the Son delivers, and the Son does not lose what the Father receives. Thus in the one kingdom there is a unity of power. Let no one therefore sever the Godhead between the Father and the Son.
Source: On the Christian Faith (De fide) (New Advent)