3 Wherefore, beloved brethren, against all the devil's deceiving snares or open threatenings, the mind ought to stand arrayed and armed, ever as ready to repel as the foe is ever ready to attack. And since those darts of his which creep on us in concealment are more frequent, and his more hidden and secret hurling of them is the more severely and frequently effectual to our wounding, in proportion as it is the less perceived, let us also be watchful to understand and repel these, among which is the evil of jealousy and envy.
And if any one closely look into this, be will find that nothing should be more guarded against by the Christian, nothing more carefully watched, than being taken captive by envy and malice, that none, entangled in the blind snares of a deceitful enemy, in that the brother is turned by envy to hatred of his brother, should himself be unwittingly destroyed by his own sword. That we may be able more fully to collect and more plainly to perceive this, let us recur to its fount and origin. Let us consider whence arises jealousy, and when and how it begins. For so mischievous an evil will be more easily shunned by us, if both the source and the magnitude of that same evil be known.
Source: The Treatises of Cyprian (New Advent)