But if some should still contend that he speaks of priests, “What sort of priests?” let us ask. Those whose walk in life has been worldly, of course. In what sense then does he say, “We shall judge angels, much more things that relate to this life?” He mentions the angels, in contradistinction to “things relating to this life”: likely enough; for they are removed from the need of these things, because of the superior excellence of their nature.

On Judgement:

Remember that the Apostle also said, We shall judge angels. And our Lord said to His disciples, Ye 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 thee. 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.

(on that source   http://www.ccel.org/ccel/schaff/npnf213.iii.iii.iii.html )