But the angels, who are the true rulers and generals and ministers of God, do not, as Celsus supposes, “injure those who offend them;” and if certain demons, whom Celsus had in mind, do inflict evils, they show that they are wicked, and that they have received no office of the kind from God. And they even do injury to those who are under them, and who have acknowledged them as their masters; and accordingly, as it would seem that those who break through the regulations which prevail in any country in regard to matters of food, suffer for it if they are under the demons of that place, while those who are not under them, and have not submitted to their power, are free from all harm, and bid defiance to such spirits; although if, in ignorance of certain things, they have come under the power of other demons, they may suffer punishment from them.
But the Christian— the true Christian, I mean— who has submitted to God alone and His Word, will suffer nothing from demons, for He is mightier than demons. And the Christian will suffer nothing, for “the angel of the Lord will encamp about them that fear Him, and will deliver them,” and his “angel,” who “always beholds the face of his Father in heaven,” offers up his prayers through the one High Priest to the God of all, and also joins his own prayers with those of the man who is committed to his keeping.
Let not, then, Celsus try to scare us with threats of mischief from demons, for we despise them. And the demons, when despised, can do no harm to those who are under the protection of Him who can alone help all who deserve His aid; and He does no less than set His own angels over His devout servants, so that none of the hostile angels, nor even he who is called “the prince of this world,” can effect anything against those who have given themselves to God.
Source: Against Celsus (New Advent)