And when they came there to the hill, behold, a company of prophets met him; and the spirit of God came upon him, and he prophesied among them.
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Augustine of Hippo
First, you ask that I explain how it can be said in the first book of Kings [Samuel], “The Spirit of the Lord came upon Saul,” when it is said elsewhere “There was an evil spirit from the Lord in Saul.” Thus it is written: “And it happened that when he turned his back to depart from Samuel, God gave Saul another heart, and all the signs came to pass on that day. Then he came to the hill and, behold, a chorus of prophets met him on the way and the Spirit of God came upon him and he prophesied among them.” But Samuel had already predicted this when he anointed him. About that, I don’t think that there is any question. For “the Spirit blows where he wills,” and no one’s soul can be fouled by contact with the Spirit of prophecy, for it extends everywhere on account of its purity. Yet, it does not affect everyone in the same way; the Spirit’s infusion in some people confers images of things, others are granted the mental fruit of understanding, others are given both by inspiration, and still others know nothing. But the Spirit works through infusion in two ways. The first way comes during sleep, and not only to saints, but even Pharaoh and King Nebuchadnezzar saw what neither of them was able to understand but both of them were able to see. The second way is through demonstration in ecstasy (which some Latins translate as “trembling”—astonishingly idiosyncratic, but close in meaning nonetheless), where the mind is separated from the bodily senses so that the human spirit, which is assumed by the divine Spirit, might be free of perceiving and intuiting ideas, as, for instance, when it was shown to Daniel what he had not understood and, to Peter, the sheet let down from heaven by its four corners, who only later recognized what this vision represented. One way is through the mental fruit of understanding, when the significance and relevance of the things demonstrated through images is revealed, which is a more certain prophecy, for the apostle calls such prophecy “greater,” as Joseph deserved to understand but Pharaoh only to see, and as Daniel explained to the king that he saw but did not know. But since the mind is affected in such a fashion that it does not understand ideas of things by conjectural examination but intuits the things themselves, as wisdom and justice and every divine form are understood to be immutable, it does not pertain to the prophecy about which we are now concerned. - "On Various Questions to Simplician 2.1.1"