16 Remember that the Apostle also said, We shall judge angels. And our Lord said to His disciples, You shall sit on twelve thrones, and judge twelve tribes of the house of Israel. And Ezekiel said concerning righteous men, that they shall judge Ahola and Aholibah. Since, then, the righteous are to judge the wicked, He has made clear concerning them that they shall not come into judgment. And as to what the apostles say, that We shall judge angels, hear, and I will instruct you. The angels who shall be judged by the apostles are the priests who have violated the law; as the Prophet said, The lips of the priest shall guard knowledge, and the law shall they inquire of his mouth; because he is the angel of the Lord, the most mighty. The angels who are the priests, of whose mouth the law is inquired, when they transgress the law, shall be judged at the last by the apostles, and the priests who observe the law.
Source: Demonstrations (New Advent)