Objection 1. It would seem that no men will judge with Christ. For it is written (John 5:22-23): "The Father... hath given all judgment to the Son, that all men may honor the Son." Therefore, etc.
Objection 2. Further, whoever judges has authority over that which he judges. Now those things about which the coming judgment will have to be, such as human merits and demerits, are subject to Divine authority alone. Therefore no one is competent to judge of those things.
Objection 3. Further, this judgment will take place not vocally but mentally. Now the publication of merits and demerits in the hearts of all men (which is like an accusation or approval), or the repayment of punishment and reward (which is like the pronouncement of the sentence) will be the work of God alone. Therefore none but Christ Who is God will judge.
On the contrary, It is written (Matthew 19:28): "You also shall sit on twelve seats judging the twelve tribes of Israel." Therefore, etc.
Further, "The Lord will enter into judgment with the ancients of His people" (Isaiah 3:14). Therefore it would seem that others also will judge together with Christ.
I answer that, To judge has several significations. First it is used causally as it were, when we say it of that which proves that some person ought to be judged. In this sense the expression is used of certain people in comparison, in so far as some are shown to be deserving of judgment through being compared with others: for instance (Matthew 12:41): "The men of Nineve shall rise in judgment with this generation, and shall condemn it." To rise in judgment thus is common to the good and the wicked. Secondly, the expression "to judge" is used equivalently, so to say; for consent to an action is considered equivalent to doing it. Wherefore those who will consent with Christ the Judge, by approving His sentence, will be said to judge. In this sense it will belong to all the elect to judge: wherefore it is written (Wisdom 3:7-8): "The just... shall judge nations." Thirdly, a person is said to judge assessorially and by similitude, because he is like the judge in that his seat* is raised above the others: and thus assessors are said to judge. [An "assessor" is one who "sits by" the judge.] Some say that the perfect to whom judiciary power is promised (Matthew 19:28) will judge in this sense, namely that they will be raised to the dignity of assessors, because they will appear above others at the judgment, and go forth "to meet Christ, into the air." But this apparently does not suffice for the fulfilment of our Lord's promise (Matthew 19:28): "You shall sit... judging," for He would seem to make "judging" something additional to "sitting." Hence there is a fourth way of judging, which will be competent to perfect men as containing the decrees of Divine justice according to which men will be judged: thus a book containing the law might be said to judge: wherefore it is written (Apocalypse 20:12): "(Judgment took her seat*) and the books were opened." [The words in brackets are not in the Vulgate. Apocalypse 20:4 we find: "I saw seats, and they sat upon them and judgment was given to them."] Richard of St. Victor expounds this judging in this way (De judic. potest.), wherefore he says: "Those who persevere in Divine contemplation, who read every day the book of wisdom, transcribe, so to speak, in their hearts whatever they grasp by their clear insight of the truth"; and further on: "What else are the hearts of those who judge, divinely instructed in all truth, but a codex of the law?" Since, however, judging denotes an action exercised on another person, it follows that, properly speaking, he is said to judge who pronounces judgment on another. But this happens in two ways. First, by his own authority: and this belongs to the one who has dominion and power over others, and to whose ruling those who are judged are subject, wherefore it belongs to him to pass judgment on them. In this sense to judge belongs to God alone. Secondly, to judge is to acquaint others of the sentence delivered by another's authority, that is to announce the verdict already given. In this way perfect men will judge, because they will lead others to the knowledge of Divine justice, that these may know what is due to them on account of their merits: so that this very revelation of justice is called judgment. Hence Richard of St. Victor says (De judic. potest.) that for "the judges to open the books of their decree in the presence of those who are to be judged signifies that they open their hearts to the gaze of all those who are below them, and that they reveal their knowledge in whatever pertains to the judgment."
Reply to Objection 1. This objection considers the judgment of authority which belongs to Christ alone: and the same answer applies to the Second Objection.
Reply to Objection 3. There is no reason why some of the saints should not reveal certain things to others, either by way of enlightenment, as the higher angels enlighten the lower [Cf. I, 106],: or by way of speech as the lower angels speak to the higher [Cf. I, 107, 2].
Source: Summa Theologica (New Advent)