6 Regarding the devil and his angels, and the opposing influences, the teaching of the Church has laid down that these beings exist indeed; but what they are, or how they exist, it has not explained with sufficient clearness. This opinion, however, is held by most, that the devil was an angel, and that, having become an apostate, he induced as many of the angels as possible to fall away with himself, and these up to the present time are called his angels.
Source: De Principiis (New Advent)