1 To be jealous of what you see to be good, and to be envious of those who are better than yourself, seems, beloved brethren, in the eyes of some people to be a slight and petty wrong; and, being thought trifling and of small account, it is not feared; not being feared, it is contemned; being contemned, it is not easily shunned: and it thus becomes a dark and hidden mischief, which, as it is not perceived so as to be guarded against by the prudent, secretly distresses incautious minds.
But, moreover, the Lord bade us be prudent, and charged us to watch with careful solicitude, lest the adversary, who is always on the watch and always lying in wait, should creep stealthily into our breast, and blow up a flame from the sparks, magnifying small things into the greatest; and so, while soothing the unguarded and careless with a milder air and a softer breeze, should stir up storms and whirlwinds, and bring about the destruction of faith and the shipwreck of salvation and of life.
Therefore, beloved brethren, we must be on our guard, and strive with all our powers to repel, with solicitous and full watch-fullness, the enemy, raging and aiming his darts against every part of our body in which we can be stricken and wounded, in accordance with what the Apostle Peter, in his epistle, forewarns and teaches, saying, “Be sober, and watch; because your adversary the devil, as a roaring lion, goes about seeking any one to devour.”
Source: The Treatises of Cyprian (New Advent)