I want to learn difference among the punishments promised to the hypocrites (munafiqs), polytheists (mushriqs), disbelievers (kafirs) and Muslims in the Hellfire by Allah Almighty. Allah has mentioned in the Quran that He, if He so wills, can forgive any sin except shirk. Does this mean that disbelieving (kufr) is also forgivable if it doesn't include any element of shirk in it? If this is true then what are these type of kufr (Like someone falsely claiming Prophethood, disbelieving in any verse of Quran, not believing in the day of resurrection etc...) And what about the hypocrites (munafiqs) who will be punished most severely in the Hell, will it be forgivable if Allah so wills!!!
First we need to appreciate the meaning of the concept of disbeliever (Kafir).
A Kafir is a person who denies the truth despite knowing it is the truth merely based on his arrogance. It is this behavior that makes one bad enough to enter the hell. The main and core element that puts some one in hell is therefore 'arrogance' that stops a person from appreciating and following the truth.
It was the same attitude that made Satan bad enough to go to hell:
"And when We stated to the Angels:" Bow down to `Adam!" So they all Bowed down save Iblis; he refused and Acted arrogantly and became one of The Kuffar." (2:34)
And it is the same attitude that makes human being bad enough to go to hell:
"Who is more wrongdoing than the one Who forges a lie against Allah or denies The Message of the Truth after it has Reached him? Is it not true that the Hell is the Home for the Kafereen?" (29:68)
'Kufr' therefore is an attitude. This attitude can apply to any groups of people. A person can be a Mushrik who might be or might not be a Kafir, or Ahl Al-Kitab who might be or might not be a Kafir (look at 98:1), or Hypocrite who might or might not be a Kafir (although by definition it seems like since arrogance is an essential ingredient of hypocrisy, in principle one whose heart is full of hypocrisy is also a Kafir - look at 4:140).
If we look at it as an abstract concept, it is even possible to say that the reason a Muslim who commits a major sin (like murder) and has not repented from it truly will go to hell is again the attitude of Kufr. This is because while as a Muslim he knows that he has committed a major sin, his arrogance stops him from true repenting, that is, he has an attitude of Kufr.
Please also bear in mind that it is not our job to decide who might be a Kafir because we normally have no idea whether one who denies the truth does it merely because of his arrogance and while he is really convinced by it.
The Qur'an does not inform us that people will stay in hell for different periods of time, according to the gravity of their sins. Rather we are informed that people will enter different parts of hell according to what they have done (look at 15:44). It seems like all the verses of the Qur'an that refer to hell, introduces it as a permanent place for those who enter it. This is because once the attitude of Kufr is established in one's heart with no possibility of true repentance, the person is not merely one who has done a mistake or has occasionally gone wrong, and the person himself becomes a manifestation of false and badness. This is why this person is sent to hell and will stay in hell permanently.
Having said that, we also need to appreciate that since the overall theme of the Qur'an is 'warning' it is most appropriate that when it refers to punishments it has to appear very strict and frightening. God knows the best but it is not unlikely that God's extensive mercy may result in much more lenience than what we expect.
Javed Ahmad Ghamidi writes in his book Mizan:
"... it can be expected that one day, Hell too shall cease to exist. The reason for this is that its torment is a threat from the Almighty and not a promise and the Almighty certainly has the right to reduce the punishment of these sinners or turn them into dust and ashes and eternally bury them in Hell" (Mizan by Ghamidi, p. 187)