Watchfulness of heart must be constant because the demons never sleep and are relentless to gain possession of what belongs to God alone. We must keep a guard over our senses, in particular, given the fact that through them we are in a constant state of receptivity. Once one is able to discern the true nature of his thoughts and how they defile, he can remain calm and peaceable even in the midst of attacks. However, no matter what level of discernment we may have achieved, the battle remains until the end of our lives. We must have no confidence in ourselves but cast ourselves upon the mercy of God.
The demons cunningly withdraw for a time in the hope that we will cease to guard our heart, thinking we have now attained peace; then they suddenly attack our unhappy soul and seize it like a sparrow. Gaining possession of it, they drag it down mercilessly into all kinds of sin, worse than those that we have already committed and for which we have asked forgiveness. Let us stand, therefore, with fear of God and keep guard over our heart, practicing the virtues that check the wickedness of our enemies.
Stand guard, then, over your heart and keep watch on your senses; and if the remembrance of God dwells peaceably within you, you will catch the thieves when they try to deprive you of it. When a man has an exact knowledge about the nature of thoughts, he recognizes those that are about to enter and defile him, troubling the intellect with distractions and making it lazy. Those who recognize these evil thoughts for what they are remain undisturbed and continue in prayer to God.
I entreat you not to leave your heart unguarded, as long as you are in the body. Just as a farmer cannot feel confident about the crop growing in his fields, because he does not know what will happen to it before it is stored away in his granary, so a man should not leave his heart unguarded as long as he still has breath in his nostrils. Up to his last breath he cannot know what passion will attack him; as long as he breathes, therefore, he must not leave his heart unguarded, but should at every moment pray to God for his help and mercy.
St. Isaiah the Solitary