We need podvigs [spiritual struggles] for the formation of our inner man, for the mortification of the passions nesting inside, to development ourselves to that "measure of the stature of the fulness of Christ" (Eph. 4:13), when we become capable of accepting and bearing the Kingdom of God in our hearts. God is always with us, at the door of our heart, as it says: "Behold, I stand at the door, and knock" (Rev. 3:20); but is the heart always able to receive Him? We are shown by books and examples that podvigs are an auxiliary force for our perfection, to purify our hearts by them, so as to be able to receive the Kingdom of God, that is, Christ Himself within it.

St. John of Kronstadt
From the book: Conversations of Archpriest John of Kronstadt with Abbess Taisia, Sretensky Monastery, 2022