When the mind, taking refuge in Christ and calling upon Him stands firm and repels its unseen enemies, like a wild beast facing a pack of hounds from a good position of defense, then it inwardly anticipates their inner ambuscades well in advance.
Through continually invoking Jesus the peacemaker against them, it remains invulnerable.
[…] Extreme watchfulness and the Prayer of Jesus Christ, undistracted by thoughts, are the necessary basis for inner vigilance and unfathomable stillness of soul, for the deeps of secret and singular contemplation, for the humility that knows and assesses, for rectitude and love.
This watchfulness and this Prayer must be intense, concentrated and unremitting.
It is written: ‘Not everyone who says to Me: “Lord, Lord” shall enter into the kingdom of heaven; but he that does the will of My Father’ (Matt. 7:21).
The will of the Father is indicated in the words: ‘You who love the Lord, hate evil’ (Ps. 97:10).
Hence we should both pray the Prayer of Jesus Christ and hate our evil thoughts. In this way we do God’s will.
Through His incarnation God gave us the model for a holy life and recalled us from our ancient fall.
In addition to many other things, he taught us, feeble as we are, that we should fight against the demons with humility, fasting, prayer and watchfulness.
For when, after His baptism, He went into the desert and the devil came up to Him as though He were merely a man, He began His spiritual warfare by fasting and won the battle by this means – though, being God, and God of gods, He had no need of any such means at all.
I shall now tell you in plain, straightforward language what I consider to be the types of watchfulness which gradually cleanse the intellect from impassioned thoughts.
In these times of spiritual warfare I have no wish to conceal beneath words whatever in this treatise may be of use, especially to more simple people.
[…] One type of watchfulness consists in closely scrutinizing every mental image or provocation; for only by means of a mental image can Satan fabricate an evil thought and insinuate this into the intellect in order to lead it astray.
A second type of watchfulness consists in freeing the heart from all thoughts, keeping it profoundly silent and still, and in praying.
A third type consists in continually and humbly calling upon the Lord Jesus Christ for help.
A fourth type is always to have the thought of death in one’s mind.
These types of watchfulness, my child, act like doorkeepers and bar entry to evil thoughts.
Hesychios the Priest (?6th-9th century): On Watchfulness and Holiness chs 8-18, Text from G.E.H. Palmer, Philip Sherrard, and Kallistos Ware (trans. and eds.) The Philokalia: The Complete Text, vol. I (Faber & Faber, London & Boston: 1979), pp. 163-165.