This life is the time appointed for people to repent:
32 For behold, this life is the time for men to prepare to meet God; yea, behold the day of this life is the day for men to perform their labors.
33 And now, as I said unto you before, as ye have had so many witnesses, therefore, I beseech of you that ye do not procrastinate the day of your repentance until the end; for after this day of life, which is given us to prepare for eternity, behold, if we do not improve our time while in this life, then cometh the night of darkness wherein there can be no labor performed.
34 Ye cannot say, when ye are brought to that awful crisis, that I will repent, that I will return to my God. Nay, ye cannot say this; for that same spirit which doth possess your bodies at the time that ye go out of this life, that same spirit will have power to possess your body in that eternal world. (Alma 34)
Death changes your body, and it changes your awareness, but it does not change your character:
And [the spirit of God] doth not dwell in unholy temples; neither can filthiness or anything which is unclean be received into the kingdom of God; therefore I say unto you the time shall come, yea, and it shall be at the last day, that he who is filthy shall remain in his filthiness. (Alma 7:21)
Death does not save you from your sins. If you have not reconciled your life to the commandments God has provided to you in this life, you will not be reconciled to God when you are resurrected.
For behold, there is a wo pronounced upon him who listeth to [disobey God in life]; for if he...dieth in his sins, the same drinketh damnation to his own soul; for he receiveth for his wages an everlasting punishment, having transgressed the law of God contrary to his own knowledge. (Mosiah 2:33)
During this life, you can repent at any time. While the massive benefits of doing this sooner rather than later include extra time to learn more about God, qualifying you for the blessings attendant with each increment, the more pressing urgency is to avoid dying in your sins. When you commit a sin—any sin—just once, you bring yourself out of alignment with God, and until you repent, you are at risk of dying out of alignment with God.
Dying in one’s sins relegates an individual to a bitter state of eternal suffering. Having rejected Jesus’ sacrifice by disobeying his commandments to them, these individuals must suffer the penalty of their own sins:
33 Wherefore, if they should die in their wickedness they must be cast off also, as to the things which are spiritual, which are pertaining to righteousness; wherefore, they must be brought to stand before God, to be judged of their works; and if their works have been filthiness they must needs be filthy; and if they be filthy it must needs be that they cannot dwell in the kingdom of God; if so, the kingdom of God must be filthy also.
34 But behold, I say unto you, the kingdom of God is not filthy, and there cannot any unclean thing enter into the kingdom of God; wherefore there must needs be a place of filthiness prepared for that which is filthy.
35 And there is a place prepared, yea, even that awful hell of which I have spoken, and the devil is the preparator of it; wherefore the final state of the souls of men is to dwell in the kingdom of God, or to be cast out because of that justice of which I have spoken.
36 Wherefore, the wicked are rejected from the righteous, and also from that tree of life, whose fruit is most precious and most desirable above all other fruits; yea, and it is the greatest of all the gifts of God. And thus I spake unto my brethren. Amen. (1 Nephi 15)
Because this is the state of those who did not repent in this life, it is incredibly important for us to reconcile ourselves to God while in this life:
33 And now, as I said unto you before, as ye have had so many witnesses, therefore, I beseech of you that ye do not procrastinate the day of your repentance until the end; for after this day of life, which is given us to prepare for eternity, behold, if we do not improve our time while in this life, then cometh the night of darkness wherein there can be no labor performed.
34 Ye cannot say, when ye are brought to that awful crisis, that I will repent, that I will return to my God. Nay, ye cannot say this; for that same spirit which doth possess your bodies at the time that ye go out of this life, that same spirit will have power to possess your body in that eternal world.
35 For behold, if ye have procrastinated the day of your repentance even until death, behold, ye have become subjected to the spirit of the devil, and he doth seal you his; therefore, the Spirit of the Lord hath withdrawn from you, and hath no place in you, and the devil hath all power over you; and this is the final state of the wicked.
36 And this I know, because the Lord hath said he dwelleth not in unholy temples, but in the hearts of the righteous doth he dwell; yea, and he has also said that the righteous shall sit down in his kingdom, to go no more out; but their garments should be made white through the blood of the Lamb. (Alma 34)
Those who opt not to repent while in life must inherit an eternal torment:
...they sell themselves for naught; for, for the reward of their pride and their foolishness they shall reap destruction; for because they yield unto the devil and choose works of darkness rather than light, therefore they must go down to hell. (2 Nephi 26:10)
The circumstances of mortality provide a special opportunity to exercise faith through repentance. After we die, we will be resurrected. As a part of what changes in resurrection, we will gain an awareness far beyond what we have here:
O how great the plan of our God! For on the other hand, the paradise of God must deliver up the spirits of the righteous, and the grave deliver up the body of the righteous; and the spirit and the body is restored to itself again, and all men become incorruptible, and immortal, and they are living souls, having a perfect knowledge like unto us in the flesh, save it be that our knowledge shall be perfect. (2 Nephi 9:13)
This additional increment of awareness makes it impossible to have the faith we can here. This is why it is not possible to repent after this life and why judgment occurs after resurrection:
43 The spirit and the body shall be reunited again in its perfect form; both limb and joint shall be restored to its proper frame, even as we now are at this time; and we shall be brought to stand before God, knowing even as we know now, and have a bright recollection of all our guilt.
44 Now, this restoration shall come to all, both old and young, both bond and free, both male and female, both the wicked and the righteous; and even there shall not so much as a hair of their heads be lost; but every thing shall be restored to its perfect frame, as it is now, or in the body, and shall be brought and be arraigned before the bar of Christ the Son, and God the Father, and the Holy Spirit, which is one Eternal God, to be judged according to their works, whether they be good or whether they be evil. (Alma 11)
Therefore, it is of the utmost importance to repent while you are still here:
Therefore, prepare ye the way of the Lord, for the time is at hand that all men shall reap a reward of their works, according to that which they have been—if they have been righteous they shall reap the salvation of their souls, according to the power and deliverance of Jesus Christ; and if they have been evil they shall reap the damnation of their souls, according to the power and captivation of the devil. (Alma 9:28)
The eternal punishment that awaits the wicked is not arbitrary. It is the natural consequence of the choices the individual has made:
30 And now remember, remember, my brethren, that whosoever perisheth, perisheth unto himself; and whosoever doeth iniquity, doeth it unto himself; for behold, ye are free; ye are permitted to act for yourselves; for behold, God hath given unto you a knowledge and he hath made you free.
31 He hath given unto you that ye might know good from evil, and he hath given unto you that ye might choose life or death; and ye can do good and be restored unto that which is good, or have that which is good restored unto you. (Helaman 14)