Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 19

Chapter 19.—That Even Among Their Own Worshippers the Name “Demon” Has Never a Good Signification.

But as some of these demonolators, as I may call them, and among them Labeo, allege that those whom they call demons are by others called angels, I must, if I would not seem to dispute merely about words, say something about the good angels.  The Platonists do not deny their existence, but prefer to call them good demons.  But we, following Scripture, according to which we are Christians, have learned that some of the angels are good, some bad, but never have we read in Scripture of good demons; but wherever this or any cognate term occurs, it is applied only to wicked spirits.  And this usage has become so universal, that, even among those who are called pagans, and who maintain that demons as well as gods should be worshipped, there is scarcely a man, no matter how well read and learned, who would dare to say by way of praise to his slave, You have a demon, or who could doubt that the man to whom he said this would consider it a curse?  Why, then, are we to subject ourselves to the necessity of explaining away what we have said when we have given offence by using the word demon, with which every one, or almost every one, connects a bad meaning, while we can so easily evade this necessity by using the word angel?