Objection 1. It would seem that this judgment, as regards the inquiry and sentence, will take place by word of mouth. For according to Augustine (De Civ. Dei xx) "it is uncertain how many days this judgment will last." But it would not be uncertain if the things we are told will take place at the judgment were to be accomplished only in the mind. Therefore this judgment will take place by word of mouth and not only in the mind.
Objection 2. Further, Gregory says (Moral. xxvi): "Those at least will hear the words of the Judge, who have confessed their faith in Him by words." Now this cannot be understood as referring to the inner word, because thus all will hear the Judge's words, since all the deeds of other men will be known to all both good and wicked. Therefore it seems that this judgment will take place by word of mouth.
Objection 3. Further, Christ will judge according to His human form, so as to be visible in the body to all. Therefore in like manner it seems that He will speak with the voice of the body, so as to be heard by all.
On the contrary, Augustine says (De Civ. Dei xx) that the book of life which is mentioned in Apocalypse 20:12, "is a kind of Divine energy enabling each one to remember all his good or evil works, and to discern them with the gaze of the mind, with wondrous rapidity, his knowledge accusing or defending his conscience, so that all and each will be judged at the same moment." But if each one's merits were discussed by word of mouth, all and each could not be judged at the same moment. Therefore it would seem that this judgment will not take place by word of mouth.
Further, the sentence should correspond proportionately to the evidence. Now the evidence both of accusation and of defense will be mental, according to Romans 2:15-16, "Their conscience bearing witness to them, and their thoughts between themselves accusing or also defending one another in the day when God shall judge the secrets of men." Therefore seemingly, this sentence and the entire judgment will take place mentally.
I answer that, It is not possible to come to any certain conclusion about the truth of this question. It is, however, the more probable opinion that the whole of this judgment, whether as regards the inquiry, or as regards the accusation of the wicked and the approval of the good or again as regards the sentence on both, will take place mentally. For if the deeds of each individual were to be related by word of mouth, this would require an inconceivable length of time. Thus Augustine says (De Civ. Dei xx) that "if we suppose the book, from the pages of which all will be judged according to Apocalypse 20, to be a material book, who will be able to conceive its size and length? or the length of time required for the reading of a book that contains the entire life of every individual?" Nor is less time requisite for telling by word of mouth the deeds of each individual, than for reading them if they were written in a material book. Hence, probably we should understand that the details set forth in Matthew 25 will be fulfilled not by word of mouth but mentally.
Reply to Objection 1. The reason why Augustine says that "it is uncertain how many days this judgment will last" is precisely because it is not certain whether it will take place mentally or by word of mouth. For if it were to take place by word of mouth, a considerable time would be necessary. but if mentally, it is possible for it to be accomplished in an instant.
Reply to Objection 2. Even if the judgment is accomplished solely in the mind, the saying of Gregory stands, since though all will be cognizant of their own and of others' deeds, as a result of the Divine energy which the Gospel describes as speech (Matthew 25:34-46), nevertheless those who have had the faith which they received through God's words will be judged from those very words, for it is written (Romans 2:12): "Whosoever have sinned in the Law shall be judged by the Law." Hence in a special way something will be said to those who had been believers, which will not be said to unbelievers.
Reply to Objection 3. Christ will appear in body, so that the Judge may be recognized in the body by all, and it is possible for this to take place suddenly. But speech which is measured by time would require an immense length of time, if the judgment took place by word of mouth.
Source: Summa Theologica (New Advent)