You may find yourself hampered by someone who sows tares of despondency. He tries to prevent you from climbing to such heights of holiness by discouraging you with various thoughts. For instance, he will tell you that it is impossible for you to be saved and to keep every single one of God’s commandments while you live in this world. When this happens you should sit down in a solitary place by yourself, collect yourself, concentrate your thoughts and give good counsel to your soul, saying: “Why, my soul, are you dejected, and why do you trouble me? Put your hope in God, for I will give thanks to Him; for my salvation lies not in my actions but in God (cf. Ps. 42:5). Who will be vindicated by actions done according to the law (cf. Gal 2:16)? No living person will be vindicated before God (cf. Ps. 143:2). Yet by virtue of my faith in God I hope that in His ineffable mercy He will give me salvation. Get behind me, Satan (cf. Matt. 16:23). I worship the Lord my God (cf. Matt. 4:10) and serve Him from my youth; for He is able to save me simply through His mercy. Go away from me. The God who created me in His image and likeness will reduce you to impotence.
Taken from the book, The Philokalia.