Someone who likes contradicting is like a man who freely surrenders himself to the enemies ranged against his emperor. The spirit of contradiction is a snare which uses self-justification as its bait, and it is by this means that we are deceived into swallowing the fishhook of sin. Then, taken by tongue and mouth, the poor soul becomes the prey of the spirits of evil. They either carry it up to the heights of pride or else cast it down into the chaos of the abyss of sin to be judged along with those who fell from heaven.
If your heart is inordinately distressed by insults and injuries, you must recognize the sign that you still carry the ancient serpent in your heart. If, then, a man keeps silent or answers back with great humility, he weakens the serpent and breaks its grip But if he replies with bitterness or speaks arrogantly, he gives the serpent the power to pour its venom into his heart and gnaw him cruelly inside. In this way the serpent grows stronger day by day, and each time the man tries to put things right it devours all the strength of his poor soul. After that he can live only for sin, and is altogether dead to righteousness.
When thoughts of despondency affect you, though you can still find zeal within your soul and so great a thirst for perfection that you long to fulfill every commandment of God, even down to avoiding the sin of an idle word, and want to equal the saints of old in virtue, wisdom, and theoria; or when you see that someone (i.e. demon) is stopping you from climbing to the heights of holiness by secretly sowing weeds of discouragement, swaying your thoughts with his comments that “it is impossible for you to be saved in the midst of this world, or even to keep the commandments of God without fault”, then go by yourself into a lonely place, recollect yourself, gather your thoughts together, and give your soul this good advice: “Why are you cast down, my soul, and why do you trouble me? Hope in God, for I shall praise Him still; for it is my God, not myself, who works my salvation. Who shall be justified by the works of the Law? No living thing shall be reckoned just in your sight, and yet, through my faith in God Himself, I hope to be saved by the grace of His ineffable compassion. So get behind me Satan, for I worship the Lord my God and Him do I serve from my youth, He who has the power to save me through His mercy alone. So get away from me! The God who made me in His own image and likeness shall make you powerless.”
—St. Symeon the New Theologian, The First Chapters