A remarkable story from the Desert Fathers:
As Abba Macarius was returning to his cell from the marsh carrying palm-leaves, the devil met him with a sharp sickle and would have struck him but he could not. He cried out: "Great is the violence I suffer from you, Macarius, for when I want to hurt you, I cannot. But whatever you do, I do and more also. You fast now and then, but I am never refreshed by any food; you often keep vigil, but I never fall asleep. Only in one thing are you better than I am and I acknowledge that." Macarius said to him, "What is that?" and he replied: "It is because of your humility alone that I cannot overcome you."As we see from this account, the demons can better even the greatest saint in external works of asceticism. Other hagiographical accounts tell us that the demons can even take on the shape of Christ, of the Mother of God, of radiant angels. They can appear to pray. They can help men tell the future and work great signs and wonders. They can control and even inhabit men; they can drive them to insanity and evil. But there is one thing within man's reach that is utterly impossible for the demons to practice: humility. Thus humility is the greatest weapon we have in resisting the assaults of the evil one and his minions.
The elders used to say: "When we do not experience warfare, we ought so much the more to humble ourselves. For God seeing our weakness, protects us; when we glorify ourselves, He withdraws His protection and we are lost."