If we are judged by God at the moment of death, why is Jesus, at his Second Coming going to judge the living and the dead at the Last Judgment?
Reasons For General Judgment
It is necessary to show why, besides the particular judgment of each individual, a general one should also be passed upon all men.
Those who depart this life sometimes leave behind them children who imitate their conduct, dependents, followers and others who admire and advocate their example, language and actions. Now by all these circumstances the rewards or punishments of the dead must needs be increased, since the good or bad influence of example, affecting as it does the conduct of many, is to terminate only with the end of the world. Justice demands that in order to form a proper estimate of all these good or bad actions and words a thorough investigation should be made. This, however, could not be without a general judgment of all men.
Moreover, as the character of the virtuous frequently suffers from misrepresentation, while that of the wicked obtains the commendation of virtue, the justice of God demands that the former recover, in the public assembly and judgment of all men, the good name of which they had been unjustly deprived before men.
Again, as the just and the wicked performed their good and evil actions in this life not without the cooperation of the body, it necessarily follows that these actions belong also to the body as to their instrument. It was, therefore, altogether suitable that the body should share with the soul the due rewards of eternal glory or punishment. But this can only be accomplished by means of a general resurrection and of a general judgment.
Next, it is important to prove that in prosperity and adversity, which are sometimes the promiscuous lot of the good and of the bad, everything is done and ordered by an all-wise and all-just Providence. It was, therefore, necessary not only that rewards should await the just and punishments the wicked, in the life to come, but that they should be awarded by a public and general judgment. Thus they will become better known and will be rendered more conspicuous to all; and in atonement for the unwarranted murmurings, to which on seeing the wicked abound in wealth and flourish in honours even the Saints themselves, as men, have sometimes given expression, a tribute of praise will be offered by all to the justice and Providence of God. My feet, says the Prophet, were almost moved, my steps had well nigh slipped, because I had a zeal on occasion of the wicked, seeing the prosperity of sinners; and a little after: Behold! these are sinners and yet abounding in the world, they have obtained riches; and I said, Then have I in vain justified my heart, and washed my hands among the innocent; and I have been scourged all the day, and my chastisement hath been in the morning. This has been the frequent complaint of many, and a general judgment is therefore necessary, lest perhaps men may be tempted to say that God walketh about the poles of heaven, and regards not the earth.
From a Catholic viewpoint, why will there be a particular judgement and a general judgment?
According to Catholicism, there will be two judgements. Catechism of the Council of Trent explains it as such:
In explaining this subject the pastor should distinguish two different occasions on which everyone must appear in the presence of the Lord to render an account of all his thoughts, words and actions, and to receive immediate sentence from his Judge.
The first takes place when each one of us departs this life; for then he is instantly placed before the judgmentseat of God, where all that he has ever done or spoken or thought during life shall be subjected to the most rigid scrutiny. This is called the particular judgment.
The second occurs when on the same day and in the same place all men shall stand together before the tribunal of their Judge, that in the presence and hearing of all human beings of all times each may know his final doom and sentence. The announcement of this judgment will constitute no small part of the pain and punishment of the wicked; whereas the good and just will derive great reward and consolation from the fact that it will then appear what each one was in life. This is called the general judgment.
The particular judgment, according to Catholic eschatology, is the divine judgment that a departed person undergoes soon after death, in contradistinction to the general judgment (or Last Judgment) of all people at the end of the world.
According to St. Augustine of Hippo (354–430 AD), the departed souls are judged as they leave the body and before the Resurrection of the Flesh.
According to the Catechism of the Catholic Church:
1021 Death puts an end to human life as the time open to either accepting or rejecting the divine grace manifested in Christ. The New Testament speaks of judgment primarily in its aspect of the final encounter with Christ in his second coming, but also repeatedly affirms that each will be rewarded immediately after death in accordance with his works and faith. The parable of the poor man Lazarus and the words of Christ on the cross to the good thief, as well as other New Testament texts speak of a final destiny of the soul—a destiny which can be different for some and for others.
1022 Each man receives his eternal retribution in his immortal soul at the very moment of his death, in a particular judgment that refers his life to Christ: either entrance into the blessedness of heaven—through a purification or immediately—or immediate and everlasting damnation.
According to the Catholic doctrine after death all "those who die in God's grace and friendship and are perfectly purified" go directly to Heaven; but "all who die in God's grace and friendship, but still imperfectly purified... they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven." Pope John Paul II affirmed that "according to Old Testament religious law, what is destined for God must be perfect". Purgatory "isn't a place, but a condition of existence" for "those who, after death, exist in a state of purification", who "removes from them the remnants of imperfection". They "are not separated from God but are immersed in the love of Christ", belonging to the Mystical Body of Christ and, by virtue of his mediation and intercession, to the Communion of Saints.
The Catholic Encyclopedia expresses that there is a need to show the merit and demerits of individuals at the end of the world and as such there will be a General Judgement.
The belief in the general judgment has prevailed at all times and in all places within the Church. It is contained as an article of faith in all the ancient creeds: "He ascended into heaven. From thence He shall come to judge the living and the dead" (Apostles' Creed). He shall come again with glory to judge both the living and the dead" (Nicene Creed). "From thence he shall come to judge the living and the dead, at whose coming all men must rise with their bodies and are to render an account of their deeds" (Athanasian Creed). Relying on the authority of Papias, several Fathers of the first four centuries advanced the theory of a thousand years' terrestrial reign of Christ with the saints to precede the end of the World. Though this idea is interwoven with the eschatological teachings of those writers, it in no way detracted from their belief in a universal world-judgment. Patristic testimony to this dogma is clear and unanimous.
The Roman Catechism thus explains why, besides the particular judgment of each individual, a general one should also be passed on the assembled world: "The first reason is founded on the circumstances that most augment the rewards or aggravate the punishments of the dead. Those who depart this life sometimes leave behind them children who imitate the conduct of their parents, descendants, followers; and others who adhere to and advocate the example, the language, the conduct of those on whom they depend, and whose example they follow; and as the good or bad influence or example, affecting as it does the conduct of many, is to terminate only with this world; justice demands that, in order to form a proper estimate of the good or bad actions of all, a general judgment should take place. . . . Finally, it was important to prove, that in prosperity and adversity, which are sometimes the promiscuous lot of the good and of the bad, everything is ordered by an all-wise, all-just, and all-ruling Providence: it was therefore necessary not only that rewards and punishments should await us in the next life but that they should be awarded by a public and general judgment."
The Last Judgment, Sistine Chapel by Michelangelo (16 c.)
The following may be of interest also:
The Short Answer, our standing before God at the time of our death is determined concerning salvation and damnation. If sanctification is not completed do to sins committed after baptism and atonement yet to be made in and through the atoning sacrifice of the Cross, that will be done before entering into the Presence of the our Father in heaven.
The General Judgment in not a separate judgment for those who have already passed, but will be a particular judgment for those who are still alive on the last day. At this time, all deeds good and bad will be made known to all so that the consequences of the actions of sin and righteousness can be made known to all.
To those whose sins are not washed by the sanctifying Grace of our Lord Jesus Christ by an obedience of Faith and Baptism, Or sanctified by some mysterious way known only to God, those persons sins will be judged for damnation and not to the Glory of God.
To those whose sins are washed by Baptism and through the Ministry of Reconciliation, which is an obedience of Faith, those sins now forgiven, will be presented to all present for the Glory of God and his Mercy.
Like Peter, when he denied Christ three times to his shame, had a full heart filled repentance before God to his Glory by showing Gods Mercy even after knowing and denying his only Son.
There is also the cascade effect that sin causes in the world and this effect will be made manifest.
Example: You are given an opportunity to defend the Catholic Faith to a person who asks you to defend it. You choose not to do so because you are perhaps intimidated by that persons own faith practice, which you know, is false. By not taking the opportunity to proclaim the Gospel as given by Christ, that person continues in his false beliefs and through generations the numbers of false disciples grows exponentially from that point on. On the Contrary, you take the opportunity to proclaim the teachings of the Gospel as preserved and continued from the time to Christ and that person remembers at some time in the future the witness you made and in part that witness leads him and generations of others to the Truth that is Christ Jesus through his Church.
This Cascade effect will be made known on the last day. Not just where evangelization and apologetics are concerned, but how every word, act of kindness, every direct sin forgiven or not, every thing that you have done which brings people to God or away from him shall be revealed.
As we continue our Christian walk as Catholics, we, like Peter who made a mistake, must grown in sanctification as we get closer to our Particular and General Judgments, we must become better Catholics, more humble more repentant sharing our faith and by so doing grow the body of Christ which is his Church.