Origen

The coming of the kingdom of God, says our Lord and Saviour, does not admit of observation, and there will be no-one to say “Look here! Look there!” For the kingdom of God is within us and in our hearts. And so it is beyond doubt that whoever prays for the coming of the kingdom of God within himself is praying rightly, praying for the kingdom to dawn in him, bear fruit and reach perfection. For God reigns in every saint, and every saint obeys God’s spiritual laws — God, who dwells in him just as he dwells in any well-ordered city. The Father is present in him and in his soul Christ reigns alongside the Father, as it is said: We will come to him and make our dwelling with him.