Envy cast an angel out of heaven, drove man out of paradise, was the first to contaminate the earth with a brother's blood, compelled brothers to sell their brother, put Moses to flight, aroused Aaron to insult his brother, defiled Miriam with jealousy toward her brother, and in short, what causes the mind to shudder, the sight to become blurred, and the hearing to fail to grasp: it aimed for and attained the very blood of Christ.
Envy is worse than all other evils: those whom it captures cannot be freed; those whom it wounds can never be cured nor return to health. Envy is the venom for offenses, the poison for iniquity, the mother of sins, the origin of the vices. The one who does not see it sees good things; the one who flees from it lives. One can avoid envy by flight, but once engaged in conflict with it one cannot win.
Sermon 48, section 5.