The main cause of warfare–arising in us through every kind of object or situation–are three: our inner disposition, the misuse of created things and, by God’s leave, the malice and onslaught of the demons. As the fallen self rises in protest against the soul, and the soul against the fallen self (cf. Galatians 5:17), so in the same way our inner disposition and our mode of acting make the passions of the fallen self war against the soul, and the valiant powers of the soul wage war against the fallen self.
And sometimes our enemy, shameless as he is, has the audacity to fight against us in his own person, without cause or warnings. Thus, my friend, do not let this blood-loving leech bleed your arteries, and then spit out the blood he has sucked from you. Do not glut the snake and the dragon, and then you will easily trample on the insolence of the lion and the dragon (cf. Psalm 91:13). Lament until you have stripped off the passions and clothed yourself in your heavenly dwelling-place (cf. 2 Corinthians 5:2), and are refashioned according to the likeness of Jesus Christ, who made you in His image (cf. Colossians 3:10).
~St Gregory of Sinai