He who has mastery over his incensive power has mastery also over the demons. But anyone who is a slave to it is a stranger to the ways of the Saviour, for as the Saviour enjoined us: 'Learn from Me; for I am gentle and humble in heart: and you will find rest for your souls' (Mt. 11:29). Now if a man abstains from food and drink, but becomes incensed to wrath because of evil thoughts, he is like a ship sailing the open sea with a demon for a pilot.
Evagrius PonticusThe Holy Spirit, out of compassion for our weakness, comes to us even when we are impure. And if He finds our intellect truly praying to Him, He enters it and puts to flight the whole array of thoughts and ideas circling within it, and He arouses it to a longing for spiritual prayer.
Evagrius Ponticus