For the kingdom of God is not in word but in power. (1 Corinthians 4:20)
St Theophan the Recluse comments on the Kingdom of God being within us:
The Kingdom of God is within us when God reigns in us, when the soul in its depths confesses God as its Master, and is obedient to him in all its powers. Then God acts within it as master ‘both to will and to do of his good pleasure‘ (Philippians 2:13). This reign begins as soon as we resolve to serve God in our Lord Jesus Christ, by the grace of the Holy Spirit. Then the Christian hands over to God his consciousness and freedom, which comprises the essential substance of our human life, and God accepts the sacrifice; and in this way the alliance of man with God and God with man is achieved, and the covenant with God, which was severed by the Fall and continues to be severed by our willful sins, is re-established. This inner alliance is sealed, confirmed, and given the strength to maintain itself by the power of grace in the divine sacrament of baptism, and for those who have fallen after baptism, in the sacrament of repentance: and afterwards it is constantly strengthened by holy communion.
All Christians live thus; and consequently they all bear the kingdom of God within themselves, that is to say that they obey God as king and are ruled by God as king.
Speaking about the kingdom of God within us, one must always add: in the Lord Jesus Christ, by the grace of the Holy Spirit. This is the mark of the Christian – the kingdom of God within us. God is the king over all, He is the Creator of all things and in His Providence watches over them all: but He truly reigns in the soul and is truly professed there as King only after the reestablishment of that union of the soul with Him which was broken by the Fall. And this union is effected by the Holy Spirit in the Lord Jesus Christ, our Savior. (THE ART OF PRAYER, pp 180-181)