St. John Cassian

Our eighth struggle is against the demon of pride, a most sinister demon, fiercer than all that have been discussed up till now. He attacks the perfect above all and seeks to destroy those who have mounted almost to the heights of holiness. Just as a deadly plague destroys not just one member of the body, but the whole of it, so pride corrupts the whole soul, not just part of it. Each of the other passions that trouble the soul attacks and tries to overcome the single virtue which is opposed to it, and so it darkens and troubles the soul only partially. But the passion of pride darkens the soul completely and leads to its utter downfall. ... But when the vice of pride has become master of our wretched soul, it acts like some harsh tyrant who has gained control of a great city, and destroys it completely, razing it to its foundations. The angel who fell from heaven because of his pride bears witness to this. He had been created by God and adorned with every virtue and all wisdom, but he did not want to ascribe this to the grace of the Lord. He ascribed it to his own nature and as a result regarded himself as equal to God.