And unclean spirits, when they saw him, fell down before him, and cried, saying, You are the Son of God.
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Augustine of Hippo
The “faith that works by love,” is not the same faith that demons have. “For the devils also believe and tremble,” but do they love? If they had not believed, they would not have said: “You are the holy one of God” or “You are the Son of God.” But if they had loved, they would not have said: “What have we to do with you?” Letter , To Sixtus.
Both the devils and the faithful confessed Christ. “Thou art Christ, the Son of the living God,” said Peter. “We know who thou art. Thou art the Son of God,” said the devils. I hear a similar confession, but I do not find a similar charity. In one there is love, in another fear. He is lovely to those who are sons. He is terrible to those who are not sons.
And unclean spirits fell down before Him, i.e, they fell clown, kneeling at His feet, not out of love and devotion, but from fear, deprecating punishment, that He would not drive them out of the men, and banish them to hell.
Saying, Thou art the Son of God. You will ask whether the devils really knew that Jesus was the Messiah or the Christ, the Son of God? I answer, it is plain from this passage, and from S. Matthew 8:29, and from S. Luke 4:41, and from the Fathers and commentators generally, that the devils, although they did not fully know Christ at His baptism, and before His baptism, because they afterwards tempted Him, that they might learn who He was; yet subsequently they did recognise who He was, from the many and great miracles, which they clearly saw were true miracles, and far transcending their own power and that of the angels. They saw that what Christ did was wrought alone by the power of God, with this end in view, that He might prove, first, that He was the Messiah ...
The unclean spirits being obliged by the Divine Power, not only to come and worship, but also to declare his majesty, exclaimed: Thou art the Son of God. How astonishing then is the blindness of the Arians, who even after his resurrection denied him to be the Son of God, whom the devils confessed as such when clothed with human nature. But it is certain that not only the devils, but the infirm that were healed, and the apostles themselves were forbidden, as well as the unclean spirits, to proclaim his divinity; lest the passion and death of Christ might be on that account deferred. (Ven. Bede)