If God is Merciful, Why is There Eternal Hell? 

Part 1: Why Does Hell Exist? 

1. First and foremost, Allah is Merciful. It is a requirement of His mercy that whoever had his rights violated in this world and did not receive any compensation there, or the criminal was not punished or was not punished as much as he should have been, then there is someone to compensate him. The mechanism would have been made. His mercy requires that He give the victim his due compensation.

It was narrated from Abu Huraira that the Messenger of Allah, may God bless him and grant him peace, said: Do you know who is bankrupt? The Companions replied: The bankrupt among us is the one who does not have a single rupee of money or equipment. On this, the Prophet (peace be upon him) said: The bankrupt of my Ummah is the one who will come on the Day of Judgment with acts like prayer, fasting and zakat. However, he may have abused one person, slandered another, eaten someone's wealth (unjustly) and shed someone's blood and killed someone. So his good deeds should be given to those people. And if his good deeds are finished and his due rights still remain, then the sins of those people will be taken and put in his account and then he will be thrown into Hell. (Sahih – Narrated by Imam Muslim, 6579)

It is also a requirement of His mercy that those whose sins are forgivable, stay in Hell as an atonement for their sins until finally they can go to Heaven after suffering the punishment they deserve.

2. Allah has many attributes besides mercy. While He is the Most Merciful, He is also Just. If everything in this world ends without any real conclusion or result, and the fate of a great person and a horrible one is the same (that they die), then it would not be equal to justice. Allah has established the system of the universe on justice, and it is the requirement of justice to reward the righteous with good and the wicked with evil. It comes in the Qur'an, "Then will We treat the Muslims like the criminals? What is [the matter] with you? How do you judge?" (Al-Qalam 35, 36).

3. God is merciful, but that mercy is not a weakness. He requires certain conditions to be fulfilled of those who are eligible for His mercy. He created Heaven to express his pleasure to those who fulfill these conditions and Hell to express his wrath to those who do not fulfill them. Heaven and Hell are the manifestation of the attributes of Allah. If we believe that imprisonment or punishment for committing crimes in this world is correct and giving a reward or some kind of bonus for a good deed is considered a logical thing, then why should the concept of punishment in the hereafter be considered strange? Obviously, when the world is a place of examination, some will pass the exam and some will fail, and those who fail will suffer the consequences.

4. A question often asked is, "Why did Allah make Hell so terrible?" Well, because otherwise people will not try to avoid it. If Hell had not been created, or if there had been a very mild punishment, people would have made no effort to avoid sin or lead a virtuous life. They would take God for granted and take the test of the world casually. Unless good is best rewarded and evil is worst punished or feared, then human psychology does not take it seriously. (Remember that no one should think that God has made an empty promise and exaggerated such things. We should take Hell and Heaven as the ultimate truth and reality that they are. Far be if from Allah to lie or make an empty promise).
It should also be understood that Allah does not express His pleasure and displeasure according to human standards, but according to His power, might and glory. Hell is the manifestation of Allah's wrath, so there cannot, and will not, be a worse place.

5. Allah has clearly stated in the Qur'an that He has no desire to send Man to Hell. He says in the Qur'an, "Why should Allah punish you if you are grateful and faithful? Allah is ever Appreciative, All-Knowing." (Al-Nisa' 147). He devised many ways to send as many people to heaven as possible. The most simple way by which every sin can be forgiven is the way of repentance. He gave hope of forgiveness for every sin, big or small, as a result of true repentance and kept the door of repentance open until death. He also stated that His mercy prevails over His wrath. He explained the methods of expiation of sins. He also said that people will be rewarded many times more for than deserved for good deeds, but the consequence for evil will only be as much as the sinner deserves.
The Prophet (peace be upon him) also said that Allah will not put His servants in Hell unless they really prove themselves worthy of it. It is narrated from Umar bin Khattab R.A., that some prisoners came to the Prophet ﷺ. Among the prisoners was a woman (she wanted to breastfeed her child and she could not find him). As soon as she found her she took the baby and started feeding it. The Holy Prophet (PBUH) said to us, "Can you imagine that this woman would put her child in the fire?" We said no, she wouldn't throw her child into the fire as long as she had the choice. The Prophet ﷺ said that Allah is more merciful to His servants than this woman is merciful to her child. (Bukhari,5653) (Muslim,2754)

Part 2: Why will Hell last forever? Why should there be an endless punishment for temporary crimes?

To understand the answer to this question, it is necessary to understand Allah (SWT)'s creation plan:

1. Allah SWT has created jinn and human beings from among His creatures with free will. They were sent to the world for the purpose of testing and this test was placed in obedience to Allah. This command of obedience was given to to jinn and humans through messengers, prophets and books. At the same time, humans and jinn were given wisdom and understanding so that they could understand these messages easily. They were also told that this exam is a very serious exam. Risking failure in this exam can result in nothing but disaster. They were also informed that they would not have another chance after death.

They were reassured that if they make any mistake during this examination, the door of repentance is open for them. Additionally, many prophets came again and again to convey this message and warnings from Allah. After all this effort and all these angles of understanding, if one still chooses the path of destruction for himself, that's his own decision. It's as if, despite multiple warning sign boards and peoples' advice, someone drives on a road that leads into a ditch. If a person chooses to be blind despite everything, then he or she is denying God's existence on purpose for no clear reason. It is as if they feel that God can be trifled with. This is against the creative plan of Allah SWT. He or she then deserves the suffering Allah SWT decides for them. Allah SWT says in Quran, "Let there be no compulsion in religion, for the truth stands out clearly from falsehood. So whoever renounces false gods and believes in Allah has certainly grasped the firmest, unfailing hand-hold. And Allah is All-Hearing, All-Knowing." (Al-Baqarah, 256)

2. Hell itself is certainly endless, but not every sinner will have to suffer eternal punishment in Hell. Those who believe in Allah and His religion, even if they commit a sin that deserves punishment, will be expelled from Hell after receiving their share of punishment. Many hadith (Prophet's sayings) show that after spending some time in Hell, Muslims who have committed major sins will be taken out from there and bathed in a canal, after which they will be purified and enter Paradise.

3. Those who will be punished eternally in Hell will be those who have truly displeased Allah. It is not so easy to offend Allah SWT as He is merciful to His creation. Even if one cannot worship Him perfectly or correctly, then He forgives, but if one does not accept Him as God, or associates with Him partners, or disdains the message of God despite being repeatedly told to him clearly, what else does he expect from the Almighty who is the Most Powerful? Of course, if you reject a clear truth with complete evidence simply for the sake of your pride or arrogance, it will make the holder of that truth (Allah SWT in this case) angry. The sins that offend Allah may be small in the eyes of us humans, but this is not for us to decide. The gravity of a sin or the scale of a good deed is all decided by Allah as He is the one who dictates the laws by which we abide. We cannot tell God based on our thoughts and our feelings what punishment He should give for what crime. Nor can we ask God why He did this and why He did not do that. He is the ruler and will decide as he likes. He is the one who determines the severity of the sin. Even if rejecting God's message is a small sin for some people, the end of the Hereafter does not depend on their thoughts and feelings.

4. Allah (SWT) is the most glorious entity in the universe. The punishment for offending him should be equally severe. It can be explained using this example: if someone kills a spider in their house, no one will care, but if that same person kills your pet, they will be punished. If a person kills their friend, that's horrible, but if they kill their mother, that's even worse! The severity of the crime often depends on how closely bonded you are supposed to be with that being. So if Allah, being the closest to us in relation, our Master, our Creator, our Guidance, our most intimate trustee is denied by us, or we upset Him in a way that He despises, then surely the punishment should match the gravity of the crime. And if for Him, this means eternal punishment, then as we established, that is His justice and His law as the Creator. In the same way, whoever pleases Allah will receive endless Heaven. If we accept the Heaven to be endless reward and believe that it is something Allah decided we deserve, then why object to Hell when it is the same principle?

5. If Allah wanted, He could have sent people to Paradise and Hell simply based on His knowledge, but He gave them a chance to live a full life so that they could be justified.We see in our day to day life that He gives everyone opportunities to turn around and improve, those who take advantage of it take it and those who don't take it, waste those opportunities. He stated in the Quran, "And your Lord is the Forgiving, full of mercy. If He were to impose blame upon them for what they earned, He would have hastened for them the punishment. Rather, for them is an appointment from which they will never find an escape." (Al-Kahaf, 58)

Allah will not punish anyone without a reason or opportunity to turn back. Therefore, no madman and no child will go to Hell. He says in Quran, "Had Allah found any goodness in them, He would have certainly made them hear. ˹But˺ even if He had made them hear, they would have surely turned away heedlessly." (Al-Anfaal, 23)

Those who will go to Hell will be those who, even if given life again and again, will repeat the same actions. In the Quran Allah SWT says, "But no! ˹They only say this˺ because the truth they used to hide will become all too clear to them. Even if they were to be sent back, they would certainly revert to what they were forbidden. Indeed they are liars!" (Al-Anaam, 28)

6. The courts of the world also do not look at the duration of the crime but look at its nature while sentencing. Killing by a bullet takes a few seconds but is usually punishable by life in prison. Likewise, denying Allah or lying to Him, if it has been the habit of one's entire life, then its end will be the punishment of the entire life of the Hereafter, and since there is no end to the life of the Hereafter, the punishment will also be eternal.

7. Allah has clearly stated in the Qur'an that He will not wrong anyone. He has forbidden oppression upon Himself. That is why when the criminals go to Hell, they will beg to be given another chance but they will not blame Allah SWT for injustice. They will either curse the deceivers who misled them from the path of Allah or they will admit their mistake. 

"And they will lament, “If only we had listened and reasoned, we would not be among the residents of the Blaze!”And so they will confess their sins. So away with the residents of the Blaze!" (Al-Mulk, 10-11)