Here is what St. John says of himself in the above-mentioned “Homily”:
After being ordained a priest and pastor, I soon learned from personal experience with whom I was entering into combat in my spiritual field of endeavor – specifically, with the strong, cunning, indefatigable prince of this world, who breathed evil, destruction and the hellish flames of Gehenna, and the spiritual host of wickedness in the heavenly places… This combat with a strong and cunning invisible enemy clearly showed me how many frailties, weaknesses, and sinful passions there were in me, how much the prince of this world had in me, and how strongly I must battle with myself, with my sinful inclinations and habits, and conquer them, in order to be as invincible as possible from the adversary’s arrows.
There began a spiritual combat, self-monitoring, the sharpening of spiritual vision, self-instruction on continuous inner prayer and the calling upon of the all-saving name of Christ; like the psalm-writer, King David, I began to constantly lift my spiritual eyes unto the hills – to heaven, from whence came obvious and immediate sovereign help (Psalm 121:1-3), and my mighty enemies were routed, while I received freedom and inner tranquility…
This warfare continues within me to this day; and the long duration of the spiritual warfare taught me much, especially the experienced perception of all intricacies of spiritual warfare, of all the numerous snares of the unseen enemies, and the firm and always assured calling upon the name of Jesus Christ, before which they are unable to stand; in this unseen warfare I came to know the constant nearness of the Lord to me, His immeasurable bounty, His quickness to hear me, the infinite holiness of His nature, for which even a single unrighteous thought is abhorrent, even an instantaneous desire for sin or pleasure in sin, since God’s holiness seeks and absolutely demands from all of us holiness in thoughts, holy zeal in feelings, holiness in all the movements of our will, in all words, in all deeds.
In this warfare I came to know the incredible depth of God’s longsuffering towards us, for He alone knows the entire frailty of our fallen nature, which He mercifully took upon Himself, except for sin, and He therefore commanded us to forgive the sins of others “sevenfold seventy times”; and He surrounded and continues to daily surround me with the joy of deliverance from sin and with inner tranquility. The mercy of God which I have experienced and the Lord’s customary nearness to me assures me of hope in my eternal salvation, and of all those who listen to me and follow my guidance towards salvation.”
St. John then speaks of how, while serving the wondrous services and communing daily of the most-holy and life-giving sacrament, he felt within himself “its life-giving nature for the soul and body, its victory over sin and death, and the feelings of redemption, peace, liberation, and fervor of the spirit which it engendered.”
Thus by means of inner endeavor and a daily serving of the Liturgy, St. John ascended from power to power, attained and was granted greater and greater gifts of the Holy Spirit. And all these gifts were abundantly poured upon St. John and testify to his holiness, his widely-known clairvoyance, miracle-working, and the countless healings he performed.
Ref: http://www.holy-transfiguration.org/library_en/saints_kronst5.html
The Art of Orthodoxy 