From the Elder Sophrony’s Wisdom from Mount Athos:
Thus the whole of spiritual warfare wages round humility. The enemy fell from pride, and would draw us to perdition by the same means. The enemy praises us, and should the soul listen to his praise grace withdraws until she repents. Thus throughout her life the soul is occupied with the lesson of Christ-like humility. So long as she has not humility wrong thoughts and impulses will always torment her. But the humble soul finds the rest and the peace of which the Lord tells.
Fasting and abstinence, vigil and withdrawal into silence, and other exploits of spiritual discipline all help, but humility is the principal power.
Humility is not learned in a trice. That is why the Lord said: ‘Learn lowliness in heart and meekness of me.’ To learn takes time. And there are some who have grown old in the practice of spiritual endeavor, yet still have not learned humility, and they cannot understand why things are not well with them, why they do not feel peace and their souls are cast down.