Objection 1. It would seem that there will not be a general judgment. For according to Nahum 1:9, following the Septuagint version, "God will not judge the same thing a second time." But God judges now of man's every work, by assigning punishments and rewards to each one after death, and also by rewarding and punishing certain ones in this life for their good or evil deeds. Therefore it would seem that there will be no other judgment.
Objection 2. Further, in no judicial inquiry is the sentence carried cut before judgment is pronounced. But the sentence of the Divine judgment on man regards the acquisition of the kingdom or exclusion from the kingdom (Matthew 25:34-41). Therefore since some obtain possession of the kingdom now, and some are excluded from it for ever, it would seem that there will be no other judgment.
Objection 3. Further, the reason why certain things are submitted to judgment is that we may come to a decision about them. Now before the end of the world each of the damned is awarded his damnation, and each of the blessed his beatitude. Therefore, etc.
On the contrary, It is written (Matthew 12:41): "The men of Nineve shall rise in judgment with this generation, and shall condemn it." Therefore there will be a judgment after the resurrection.
Further, it is written (John 5:29): "They that have done good things shall come forth unto the resurrection of life, but they that have done evil, unto the resurrection of judgment." Therefore it would seem that after the resurrection there will be a judgment.
I answer that, Just as operation refers to the beginning wherefrom things receive their being, so judgment belongs to the term, wherein they are brought to their end. Now we distinguish a twofold operation in God. One is that whereby He first gave things their being, by fashioning their nature and by establishing the distinctions which contribute to the perfection thereof: from this work God is stated to have rested (Genesis 2:2). His other operation is that whereby He works in governing creatures; and of this it is written (John 5:17): "My Father worketh until now; and I work." Hence we distinguish in Him a twofold judgment, but in the reverse order. One corresponds to the work of governance which cannot be without judgment: and by this judgment each one is judged individually according to his works, not only as adapted to himself, but also as adapted to the government of the universe. Hence one man's reward is delayed for the good of others (Hebrews 11:13-40), and the punishment of one conduces to the profit of another. Consequently it is necessary that there should be another, and that a general judgment corresponding on the other hand with the first formation of things in being, in order that, to wit, just as then all things proceeded immediately from God, so at length the world will receive its ultimate complement, by each one receiving finally his own personal due. Hence at this judgment the Divine justice will be made manifest in all things, whereas now it remains hidden, for as much as at times some persons are dealt with for the profit of others, otherwise than their manifest works would seem to require. For this same reason there will then be a general separation of the good from the wicked, because there will be no further motive for the good to profit by the wicked, or the wicked by the good: for the sake of which profit the good are meanwhile mingled with the wicked, so long as this state of life is governed by Divine providence.
Reply to Objection 1. Each man is both an individual person and a part of the whole human race: wherefore a twofold judgment is due to him. One, the particular judgment, is that to which he will be subjected after death, when he will receive according as he hath done in the body [Cf. 2 Corinthians 5:10, not indeed entirely but only in part since he will receive not in the body but only in the soul. The other judgment will be passed on him as a part of the human race: thus a man is said to be judged according to human justice, even when judgment is pronounced on the community of which he is a part. Hence at the general judgment of the whole human race by the general separation of the good from the wicked, it follows that each one will be judged. And yet God will not judge "the same thing a second time," since He will not inflict two punishments for one sin, and the punishment which before the judgment was not inflicted completely will be completed at the last judgment, after which the wicked will be tormented at the same time in body and soul.
Reply to Objection 2. The sentence proper to this general judgment is the general separation of the good from the wicked, which will not precede this judgment. Yet even now, as regards the particular sentence on each individual, the judgment does not at once take full effect since even the good will receive an increase of reward after the judgment, both from the added glory of the body and from the completion of the number of the saints. The wicked also will receive an increase of torment from the added punishment of the body and from the completion of the number of damned to be punished, because the more numerous those with whom they will burn, the more will they themselves burn.
Reply to Objection 3. The general judgment will regard more directly the generality of men than each individual to be judged, as stated above. Wherefore although before that judgment each one will be certain of his condemnation or reward, he will not be cognizant of the condemnation or reward of everyone else. Hence the necessity of the general judgment.
Source: Summa Theologica (New Advent)