Short Answer: Eternal hell is reserved for people who have committed the only unforgiveable sin: shirk, or worshiping anyone or anything instead of or alongside God Himself. But other believers who have not repented of their major sins may spend a duration in Hell to purify themselves of their sins before being admitted into Paradise. And Allah knows best who is a true believer.
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We would like to note here that your questions are indeed very critical; they also require authentic fiqhi/scholarly opinion because matters of the afterlife, heaven and hellfire need to be dealt with carefully and with full knowledge of the Share’aa, the hadiths and the noble Qur’an.
So, we advise you to contact our Ask the Scholar section as well to get a detailed scholarly opinion that reassures you even further.
We will attempt to answer your questions generally here as well, but it is still best to ask a scholar.
Who enters Heaven?
Allah Glorified and Exalted is Just and He is also the Most Merciful.
Allah has already made the path clear for us and sent us messengers and the final Messenger to teach us, give glad-tidings and warn the disbelievers and wrongdoers.
There are no surprises or injustices in this path.
But he who does of righteous deeds while he is a believer – he will neither fear injustice nor deprivation. (Qur’an 20:112)
We know that the key to enter heaven eternally is the sincere belief in Allah Almighty with no partners, and the good deeds that are done purely for His sake.
And whoever does righteous deeds, whether male or female, while being a believer – those will enter Paradise and will not be wronged, [even as much as] the speck on a date seed. And who is better in religion than one who submits himself to Allah while being a doer of good and follows the religion of Abraham, inclining toward truth? And Allah took Abraham as an intimate friend. (Qur’an 4:124-125)
But What Is a True Believer?
Someone is called a “true believer” who humbly submits to Allah and believes in His Wisdom and what He taught His Messenger.
But no, by your Lord, they will not [truly] believe until they make you, [O Muhammad], judge concerning that over which they dispute among themselves and then find within themselves no discomfort from what you have judged and submit in [full, willing] submission. (Qur’an 4:65)
However, some people claim to believe, but they are not actually sincere and/or they associate others with Allah Almighty.
If someone claims to be a Muslim, yet he/she doesn’t see a problem in sinning and do not repent, then Allah tells us that these are not the true believers, even if they say they are Muslims who believe with their tongue, but Allah knows what’s truly in their hearts.
Likewise, if someone rejects the main parts of the faith deliberately, as you mentioned, they are not a true believer, according to Allah.
Will Muslims dwell in hell eternally?
If someone is a true Muslim who sincerely believes in Allah Almighty, but insisted on those major sins without repentance, then this matter is up to Allah Almighty.
The Prophet (peace and blessings upon him) said, “Whoever said “None has the right to be worshipped but Allah and has in his heart good (faith) equal to the weight of a barley grain will be taken out of Hell. And whoever said: “None has the right to be worshipped but Allah and has in his heart good (faith) equal to the weight of a wheat grain will be taken out of Hell. And whoever said, “None has the right to be worshipped but Allah and has in his heart good (faith) equal to the weight of an atom will be taken out of Hell.” [Sahih Al Bukhari]
Ultimately, someone who insists on major disobedience of Allah Almighty without repenting, then his/her faith is not big or strong.
But he/she could have some faith, and on account of that faith—no matter how little it is, as you see in the hadith—they will get out of Hell and not dwell there eternally.
A Believer is Not Like a Disbeliever
But the key is that Allah Almighty will not equate the ones who obeyed and the ones who disobeyed.
Then is one who was a believer like one who was defiantly disobedient? They are not equal.” (Qur’an 32:18)
Just think about it: if the students who studied and did well in the exam eventually got the same grade as those who didn’t study and didn’t perform well in the exam, is this justice?
Allah is Just.
If He allows major sins and disobedience with no warning or account for that, then this means we are allowing corruption, ignorance, and darkness to fill the earth as there is no need to strive for good deeds since everyone will equally enter heaven.
Who/What earns eternal hell?
No one will abide forever in hell except the disbelievers who insisted on disbelieving and denying their Creator Allah Almighty.
He gave them this life for the purpose of knowing Him and worshipping Him, and they missed the very purpose of their existence.
So, the sin that makes one dwell in hell eternally is shirk (associating others with Allah Almighty).
Those who get the worst punishment are those who purely disbelieved in Allah Almighty without even the ounce of sincere faith in Him.
Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly fabricated a tremendous sin. (Qur’an 4:48)
Hope this answers your questions; and please do contact Ask The Scholar as well.