All that belongs to God carries the seal of immortality. And, the Kingdom of God is immortal. If we desire to breathe the air of immortality, we must enter within ourselves, within our hearts, within the Kingdom of God. Outside of ourselves is the air of time, the air of transitoriness and decay in which the soul breathes with difficulty. … We are constantly outside ourselves. The Lord wants to return us to ourselves, in His home and to His homeland. For us, the Kingdom of God is within us: outside of ourselves is a foreign land. In order to escape from a foreign land and find our true home, in which we directly encounter God, we must enter within ourselves, into our hearts. There is the King, there also is the Kingdom.
St. Nikolaj (Velimirovic)
Just like the kingdom of God, the Spirit has both a gradual historical manifestation and a discontinuously eschatological manifestation, which are irreducible one to the other. Otherwise it is incomprehensible how the final state, the illumination of creation the expulsion of death, in a word, the ‘future age’ could be distinguished from the preliminary state of waiting, from ‘this age,’ in which death still reigns.
Thus, the ideas of the Kingdom of God and of the Holy Spirit resemble each other formally. But this resemblance is not only formal. In its general idea, the doctrine of the Holy Spirit as the Kingdom of the Father unquestionably has its roots in the Gospel, and it gets its verbal justification in the Apostle Paul: ‘The kingdom of God is righteousness, and peace, and joy in the Holy Spirit’ (Rom. 14:17)—Εν Πνευματι Αγιωι, ‘in’ or ‘of the Holy Spirit,’ i.e., in the righteousness, peace, and joy produced by the Holy Spirit. The subjective state of righteousness, peace, and joy produced by the Holy Spirit is that same Kingdom of God which is ‘within’ us (Luke 17:21), the barely noticeable mustard seed of faith sown in the soul. But growing and showing itself above the field of what is mine and only mine, above the domain of subjectivity, the sprout of the seed of faith becomes objective, cosmic, universal. Liturgy and the sacraments are the outward manifestations of the Kingdom of God in Church life. The working of miracles and contemplative insights reveal the same Kingdom in the personal lives of the saints.
Fr. Pavel Florensky
The habit of virtue is the restoration of the powers of the soul to their primordial nobility and the unification of the main virtues for action that is proper to the soul by its nature. This does not come to us from outside, but is innate to us from our creation, and through this we enter into the kingdom of heaven, which according to the Lord’s words is within us.