Many (monks) are not aware how the demons deceive the intellect. Being naive and undeveloped, they tend to give all their attention to the practice of the virtues and do not bother about the intellect. They move through life, I fear, without having tasted purity of heart, and are totally ignorant of the darkness of the passions within. Such people, unaware of the battle about which Paul speaks (Ephesians 6:12) and not imbued with personal experience of true goodness, regard as lapses only those sins which are actually put into effect. They do not take into account the defeats and the victories that occur on the plane of thought, for these, being internal, cannot be seen by natural sight and are known only to God our judge, and to the conscience of the spiritual contestant. I take it that the scriptural words, ‘They said, “Peace”, but there was no peace’ (Ezekiel 13:10), apply to such people. The other brethren pray for them in their simplicity, and as best they can teach them to avoid the actual commission of sin. But for those who have a divine desire to cleanse the vision of the soul there is another form of activity in Christ and another mystery.
~St Philotheos of Sinai
Prayerful Life 