Take advantage of St. Ignatius Loyola's guide to discerning spirits.
Well, St. Ignatius Loyola, a Catholic priest and founder of the Jesuits (the Society of Jesus), has the answer. He reveals this in his teaching of the discernment of spirits, Aleteia reports.
According to him, you can tell by the kind of thoughts you have in your head.
He writes, "If in the course of the thoughts which he brings it ends in something bad, of a distracting tendency, or less good than what the soul had previously proposed to do, or if it weakens it or disquiets or disturbs the soul, taking away its peace, tranquillity, and quiet, which it had before, it is a clear sign that it proceeds from the evil spirit, enemy of our profit and eternal salvation."
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St. Loyola further explains that you can also tell by the state of one's soul. He writes that if the soul is moving from "good to better" the good Angel touches "sweetly" while the bad Angel touches, "sharply and with noise and disquiet."
If the reverse is the case and the soul is moving from "bad to worse" then the bad Angel will try to lure us into even worse sin, "sweetly," while the good spirit attempts to pint us the right direction.
His tips have been confirmed by a 16th-century priest named Dom Lorenzo Scupoli who wrote, "The devil does his utmost to banish peace from one's heart because he knows that God abides in peace and it is in peace that He accomplishes great things."
So, the next time you hear a voice in your head, make sure you take advantage of St. Loyola's guide to discerning spirits.