It is mentioned that, in the Hereafter, the teeth of deniers become as big as the mountain of Uhad (then like what will be his body? 
. In other word, in Aakherat, meaning: (reality) will overcome face (appearance). All facts will come out in the open and the inner and outer will become manifest.
Yawma Tublas Saraait (Surah 86:9)
Qiyamat is the Day of the unveiling of secrets. It is also mentioned that when the inner and outer organs of body will disintegrate and tear up by chastisement (such as eating Zaqqoom and drinking Hameem) all these body parts will recover soon again (rather the skin will be changed a number of times).
The Holy Qurans mentions clearly: Kul Lamaa…Azaab (Surah Nisaa 4:59) meaning: Whenever their skins will burn up We will change them so that they may continue to taste the chastisement adequately.
Is Painful Punishment Becoming Divine Justice?
Another doubt which can come to man's mind is: Are such severe punishments communstrate with Divine Justice?
Such doubt is the result of thinking that the Divine punishment is like the tortures inflicted by the despotic rulers of this world. But this imaginary likeness is in no way correct because what the worldly tyrants to is for taking personal revenge and for pleasing themselves by punishing their offenders. They want to cool their anger in that way. They frown upon those who oppose them; they boil in anger. In order to subside their wrath they order their imprisonment, torture and even killing etc. But the Divine punishment will not be like that. There, whatever will happen will be caused by what had happened in the world. If the sin is serious its punishment will also be serious and if the error was light its punishment will also be proportionately light. The result of faithlessness will be to remain in perpetual darkness Turning away from God will result in permanent sandwitching and pressure. The result of every trampled right will be everlasting blindness and deafness. The consequence of following devilish leaders will be dwelling with them forever. The essential result of eating the property of an orphan will be flaring of fire in the devilish devourer's stomach. This makes it clear that man will get hell fire by way of his own earning. He will be liable to various punishments in hell only because of his own misdeeds and evil character and misbehaviour. In the light of what is said in the Holy Quran the fuel of hell fire also will be wrongdoer persons: Fattaquo…Hijaarah. (Surah Baqarah 2: 24)
Self Prepared Punishment
It can be understood from the above that the reasons of getting punishments in hell will have been provided by the sinner man himself. He had behaved wrongfully unjustly and oppressively with himself. God never oppresses anyone: Innallaah La…Yazlimoon. (Surah Yunus 10: 44)
How at all a Kafir denier ever be given entrance to paradise which is extremely Holy and soft place when that fellow had indulged in dirtiness, harshness, rebellion, deviation and disobedience to God?! Bihisht (Paradise) is a very very secure and peaceful place where only healthy people can enter who does not have any foul illnss. How can a man whois full of internal illnesses like envy, malignness, enmity, stinginess, pride, egotism, be allowed engrance to paradise? The residence of such ill people is a hospital (Beemaaristaan) of hell, not the paradise which is a place of permanent peace.
Adoration Before The Blind And Singing Before The Deaf?!
The truth is that to allow a denier entrance to paradise is just like pouring scented oil on the head of a man who has lost his faculty of smelling due to severe cold. It is like putting a thing at an inappropriate place. (It is never commensurate with Divine Wisdom). It is called Zulm in Arabic. Therefore hurling a Kafir in hell is in no way opposite of Divine Justice because in Arabic ADL means to put a thing in its appropriate place (God is ADL means Just). That is why entrance of a denier in paradise is impossible. It is totally against divine justice (Adl-e-Ilaahi). He (the Kafir) can never enjoy paradise because it is just like a beautiful bride who adorns herself for a blind fellow or sings before a deaf person.
What Has The Merciful To Do With Punishment?!
Even after all that as been said above, it is likely that some one may ask: How can God, the Most Merciful, Who loves His creation beyond measure, will put his beloved creation to fierce punishments?! The answer is that though God is most soft and lenient, He also is wrathful and mighty. As He is Rehman and Raheem, so also He is Jabbaar and Qahhaar. Just as paradise is the manifestation of extreme kindness, so is the hell the manifestation of extreme anger and wrarh. Yaa man Sabaqat Rahmatahu Ghazabahu is the attribute according to which He is not happy that His creation should become Kafir by disobeying Him and go to hell. (Laa Yarzaa Li Ibadihil Kufr). But when the creation, despite knowing His displeasure, opts for the road leading to hell, He also does not prevent them per force from going in that direction because it is against His wisdom God is absolutely independent of every or any need. He is also not needy of our faith.
Forcible Tawbah (Repentance) Is Useless
Yet He has, on the basis of His mercy and kindness, towards His creation made Tawbah (true repentance) the cause of protection from the above mentioned punishments. He has kept this door open until the last breath of man. But if the creation turns away its face from this kindness also, He also does not force them to repent because a forced tawbah will not be a tawbah at all. Doubtlessly, God is Merciful and Beneficent and it is because of it that He has ordered the creation to follow His commands and to carry out all obligatory and voluntary good deeds leading to paradise and He guided us toward it. Similarly He also commanded us to refrain from all evil, prohibited and indecent things which take one away from God and make him near to Satan and consequently lead to hellfire. Thus it can be understood that all these religious commands and dutied are due to His vast mercy and kindless. Therefore, a faithful person should also take care and he should never lag behind in fulfilling his religious duties. He must realize that failure to do any obligatory duty is his permanent loss which is irrepartable. In this way he will be depriving himself of divine mercy. No other thing can avert this loss.
A Short Lifespan And Permanent Punishment
One more objection can also be that if a Kafir got a lifespan of say fifty or sixty (or more) years. Then, how can it be proper that he should suffer for over ever endlessly in hell? The answer is: The denier who, spent his entire life in disbelief had, in reality, in that very short lifespan by misusing his freedom of action, gathered such a huge and permanent evil that its essential consequence is entrance to everlasting hell. In other words, he opted for such a darkness for himself, where no ray of light can ever reach till eternity. Likewise, the faithful who left this world with piety and God fearingness earnerd in his short life and undiminishing success, that is, he earned and inextinguishable light.
Eternity Will Be Based On The Intention Of Good Or Evil
It is mentioned in Biharul Anwaar that Abu Hashim asked Imam Sadiq (a.s.) why the dwelling in paradise or hell will be permanent? The Imam (a.s.) replied: The people of hell will live in hell forever because their intention in the world was that had they to live in the world forever they would disobey God forever. Likewise the reason of the people of paradise living there in permanently is also that their intention in the world was that if they had to live in the world forever they would always obey God and and would never defy his commandments. So, it is the intention that results in the permanent residents of the people of paradise in paradise and the permanent dwelling if the people of hell in hell. The Holy Quran says: Everyone acts according to his intention.