The kingdom of God, in the words of our Lord and Savior, does not come for all to see; nor shall they say: Behold, here it is, or behold, there it is; but the kingdom of God is within us, for the word of God is very near, in our moth and in our heart. Thus it is clear that he who prays for the coming of God’s kingdom prays rightly to have it within himself, that there it might grow and bear fruit and become perfect. For God reigns in each of his holy ones. Anyone who is holy obeys the spiritual laws of God, who dwells in him as in a well-ordered city. The Father is present in the perfect soul, and with him Christ reigns, according to the words: We shall come to him and make our home with him.
Thus the kingdom of God within us, as we continue to make progress, will reach its highest point when the Apostle’s words are fulfilled, and Christ, having subjected all his enemies to himself, will hand over his kingdom to God the Father, that God may be all in all. Therefore, let us pray unceasingly with that disposition of soul which the Word may make divine, saying to our Father who is in heaven: Hallowed be your name; your kingdom come.
Note this too about the kingdom of God. It is not a sharing of justice with iniquity, nor a society of light with darkness, nor a meeting of Christ with Belial. The kingdom of God cannot exist alongside the reign of sin.
Therefore, if we wish God to reign in us, in no way should sin reign in our mortal body; rather we should mortify our members which are upon the earth and bear fruit in the Spirit. There should be in us a kind of spiritual paradise where God may walk and be our sole ruler with his Christ. In us the Lord will sit at the right hand of that spiritual power which we wish to receive. And he will sit there until all his enemies who are within us become his footstool, and every principality, power and virtue in us is cast out.
All this can happen in each one of us, and the last enemy, death, can be destroyed; then Christ will say in us: O death, where is your sting? O hell, where is your victory? Ans so what is corruptible in us must be clothed with holiness and incorruptibility; death will be cast out, and our mortality will be clad with the Father’s immortality, so that, as God reigns in us, we may truly enjoy the blessings of rebirth and resurrection.
Origen, Notebook On Prayer (Cap. 25: PG 11, 495-499)