pasted from http://www.johnsanidopoulos.com/2010/03/coming-judgment.html:
In the Biblico-patristic tradition we also see another way in which men will be judged in the future Judgement. It is said that men will be judged by the saints. We find this already in Christ's words to His disciples: "Assuredly I say to you, that in the regeneration, when the Son of Man sits on the throne of His glory, you who have followed Me will also sit on twelve thrones, judging the twelve tribes of Israel” (Matt. 19:28).
The Apostle Paul maintains the same thing. Reproaching the Christians for turning to worldly tribunals to solve their various affairs, he says: "Do you not know that the saints will judge the world?” (1 Cor. 6:2). But how is this judgement known by the saints?
St. Symeon answers this point as well. He says that every man, finding himself faced with eternal life and that unutterable light, will see "one who is like him and will be judged by him". All men who have lived on earth in different ways of life will be judged by other men who have lived with them in the same conditions of life. And the ones lived in accord with the will of God, the others rejected His commandments. This means that there can be no excuse that the conditions of life were difficult and that therefore they could not live according to God's ordinances.
Thus fathers will be judged by fathers, relatives and friends by relatives and friends, brothers by brothers, the rich by those who were rich, the poor by those who were poor, the married by those who have excelled in the married state, etc. When sinners look at sinners who have repented, whoremongers who have not repented see penitent whoremongers, when the kings see holy kings, etc, and in general, when each person sees that someone like himself, who had the same nature, the same hands and eyes, the same conditions of life has been saved, this will be a self-condemnation, he will have no arguments and no excuses.
St. Symeon's words which I shall quote exactly are very characteristic: "Thus each of us sinners will be condemned by each of the saints, and likewise unbelievers by those who believe, and sinners who have failed to repent by those who perhaps have sinned more but have fervently repented".
It is terrible at that hour for someone to see in the glory of God "him who received the tonsure with him standing on the right hand, the one who ate and drank with him, his contemporary, his colleague” being completely surrounded by great glory like Christ, while he himself is the opposite. Then he will be unable to speak at all.
This is just what it means that we shall be judged by the saints. We will be censured by their penitence and by the fact that they lived under the same conditions and yet they have been shown to be recipients of the Holy Spirit, imitators of Christ in every respect.