If the devil has been driven out and sin no longer reigns, then the kingdom of God is established in us. As it is written in the Gospel, “The kingdom of God does not come with observation, nor will they say, ‘Lo, here,’ or ‘Lo, there.’ Truly I say to you that the kingdom of God is within you.” The only thing that can be “within us” is knowledge or ignorance of the truth and the affection for righteousness or sin by which we prepare our hearts to be a kingdom of Christ or the devil. St. Paul described the nature of this kingdom in this way: “For the kingdom of God is not food and drink, but righteousness, peace and joy in the Holy Spirit.” If the kingdom of God is within us and is righteousness, peace and joy, then someone that remains in these is surely within the kingdom of God. Someone that remains in unrighteousness, conflict and the melancholy that kills the life of the spirit is already a citizen of the devil’s kingdom, of hell and of death. These are the signs whether it is God’s kingdom or the devil’s.
John Cassian, Conference 13, quoted in Arthur A Just (ed), Luke, Ancient Christian Commentary on Scripture, New Testament III, (Intervarsity Press, 2003) 271.
A man is not freed from the pleasure of sin’s working until he truly abhors the cause of sin with his whole heart. This is the fiercest struggle, the struggle that withstands a man unto blood, wherein his free will is tested as to the unity for the love of the virtues. This is the power which some call enticement and pitched battle, and by the scent of it the wretched soul is enfeebled because of the intense provocation which lies therein. This is the mighty power of sin by which the enemy is wont to trouble the souls of the chaste and to compel the pure movements [of their souls] to experience what they had never in any wise experienced. It is here that we manifest our patience, my beloved brethren, our struggle and our zeal. For this is the time of unseen contest, in which it is said that the monastic order always excels. If the upright intellect does not securely guard itself, it will speedily be confounded by its encounter with this warfare.
St. Isaac the Syrian,The Ascetical Homilies of Saint Isaac the Syrian (I, 32), translated by the Holy Transfiguration Monastery, Boston, 1984. p. 150