And Christians have nothing to fear, even if demons should not be well-disposed to them; for they are protected by the Supreme God, who is well pleased with their piety, and who sets His divine angels to watch over those who are worthy of such guardianship, so that they can suffer nothing from demons. He who by his piety possesses the favour of the Most High, who has accepted the guidance of Jesus, the “Angel of the great counsel,” being well contented with the favour of God through Christ Jesus, may say with confidence that he has nothing to suffer from the whole host of demons. “The Lord is my light and my salvation; whom shall I fear? The Lord is the strength of my life; of whom shall I be afraid? Though an host should encamp against me, my heart shall not fear.” So much, then, in reply to those statements of Celsus: “If they are demons, they too evidently belong to God, and they are to be believed, to be sacrificed to according to the laws, and prayers are to be offered to them that they may be propitious.”
Source: Against Celsus (New Advent)