The kingdom of God is within us!
Everything depends on the inward frame of mind, and when the devil has been expelled from this, and sins no longer reign in it, it follows that the kingdom of God is founded in us
[Abba Moses said:] “For everything depends on the inward frame of mind, and when the devil has been expelled from this, and sins no longer reign in it, it follows that the kingdom of God is founded in us, as the Evangelist says "The kingdom of God cometh not with observation, nor shall men say Lo here, or lo there: for verily I say unto you that the kingdom of God is within you." (Lk 17:20-21) But nothing else can be "within you," but knowledge or ignorance of truth, and delight either in vice or in virtue, through which we prepare a kingdom for the devil or for Christ in our heart: and of this kingdom the Apostle describes the character, when he says "For the kingdom of God is not meat and drink, but righteousness and peace and joy in the Holy Ghost." (Rom 14:17) And so if the kingdom of God is within us, and the actual kingdom of God is righteousness and peace and joy, then the man who abides in these is most certainly in the kingdom of God, and on the contrary those who live in unrighteousness, and discord, and the sorrow that worketh death (cf. 2 Cor 7:10), have their place in the kingdom of the devil, and in hell and death. For by these tokens the kingdom of God and the kingdom of the devil are distinguished.”
St John Cassian, Conferences I,13