According to Christian demonology the mission of the demons is to tempt humans inducing them to sin, proving their spiritual strength or testing their faith in God.
Demons have also the duty of punishing the souls of those people that died out of God's grace, this means in sin, torturing them in Hell for all the eternity.
Tormenting people during their life, like the case of Job or by possessing them, causing disgraces and diseases, or simply showing themselves before persons to afraid them, provoking visions that could induce people to sin or to be afraid, are things believed to be their work too.
Another of their works is trying to make people abandon the "true" faith (Christianity), commit heresy and/or apostasy, remain or turn themselves Pagan or idolatrous and gain the highest number of "satans" or adversaries of God.
All evilness in this world is allegedly caused by demons with God's permission, according both to Christian theology and Christian demonology, as their mission serving God (Matthew 4:10 and Luke 4:8).
The following is an early Christian rhyme, attributed to St. Benedict, thought effective to avoid temptations and inhibit Satan's power (by that time Satan was the only name of the chief demon):
This rhyme is still used by some Catholics, especially in the form of the initials of its words (VRS:NSMV:SMQL:IVB) engraved on a cross. As a superstition the first sentence (Vade retro Satana) is used to repel any possible evil thing or happening, as a "spoken amulet".
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