I know that non-muslims are going to be denied paradise, but my question is what is the justification behind their disbelief being worthy of eternal torment?
To Allah belongs whatever is in the heavens and whatever is in the earth. Whether you show what is within yourselves or conceal it, Allah will bring you to account for it. Then He will forgive whom He wills and punish whom He wills, and Allah is over all things competent. —Qur'an, 2:284
❶ - Those who disbelieved did so for the entire life span, so they will be punished for the entire life span in the next life. Even though this life span is finite, it does not refute the fact that they disbelieved for as long as they lived, and would have disbelieved for as long as they could live. And in fact, if they were returned to this world, they would continue disbelieving. So the just punishment is to be punished in the next life for as long as they will live in the next life (forever).
ولو ترى إذ وقفوا على النار فقالوا يا ليتنا نرد ولا نكذب بآيات ربنا ونكون من المؤمنين - بل بدا لهم ما كانوا يخفون من قبل ولو ردوا لعادوا لما نهوا عنه وإنهم لكاذبون - "If you could but see when they will be held over the (Hell) Fire! They will say: "Would that we were but sent back (to the world)! Then we would not deny the Ayât (proofs, evidences, verses, lessons, revelations, etc.) of our Lord, and we would be of the believers!" Nay, it has become manifest to them what they had been concealing before. But if they were returned (to the world), they would certainly revert to that which they were forbidden. And indeed they are liars." (6:27-28)
❷ - They chose eternal punishment. Disbelievers chose to be punished for the entire Hereafter, because they were warned by God and repeatedly informed of the punishment for disbelief. They knew it, and chose it, and in fact, you find many of them in the zeal of ignorance demanding God bring on the punishment, as a way of mockery and challenge to God. So, they chose it.
ويستعجلونك بالعذاب ولولا أجل مسمّى لجاءهم العذاب وليأتينهم بغتة وهم لا يشعرون - يستعجلونك بالعذاب وإن جهنم لمحيطة بالكافرين - "And they ask you to hasten on the torment (for them), and had it not been for a term appointed, the torment would certainly have come to them. And surely, it will come upon them suddenly while they perceive not! They ask you to hasten on the torment. And verily! Hell, of a surety, will encompass the disbelievers." (29:53-54)
Unfortunately you've misunderstood. There is no evidence from the Quran that every non muslim will 100% go to Hell. Is Allah unjust? No.
Those who believe and those who are Jews and the Sabaeans and the Christians, all who believe in Allah and the Last Day and act rightly will feel no fear and will know no sorrow. (Surat al-Maida, 69)
Among the People of the Book (Jews and Christians) there are some who believe in Allah and in what has been sent down to you and what was sent down to them, and who are humble before Allah. They do not sell Allah’s Signs for a paltry price. Such people will have their reward with their Lord. And Allah is swift at reckoning. (Surah Al ‘Imran, 199)
If the Non Muslim believes in one God, and the Last day, and obviously does more good deeds than bad, he/she is eligible to enter paradise.
Allah is the Most Just, he won't just ignore all your good deeds just because you're not a muslim.
I am not putting evidence here but as original post demanded reasoning, I want to tell my understanding. God has created everything for His recognition (in Arabic, it is called maarefat). If He is the God and let you enter in paradise without knowing Him, denies the purpose of the creation of World.
In the name of God, the Most Beneficent, Most Merciful.
First of all, let me tell you something, no one knows exactly 100% what will get on the Day of Judgment, only God knows perfectly because God Himself Says:
”[He is] Knower of the Ghaeeb [future and unseen] and the witnessed, the Grand, the Exalted.”[13:9]
”Indeed, God [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, God is Knowing and Acquainted.” [31:34]
According to the those verses above, it tells that you cannot know exactly if non Muslims will enter Paradise or not, you just say like this, but you don’t know for sure because only He knows.
And what do you say about people who lived thousands and thousands of years before Islam came and before prophet Muhammad? And there were good and bad people, would they be punished by God just because they never heard even the word “Islam”? Would they be Punished? No, if God is Most Generous, the Most Kind, the Most Just and so on To His creation, why would He punish someone who never heard the true message in its form or never heard about nothing of Islam? That would be unjust if He would punish someone who never heard of Islam or heard of it but has misunderstood it, because, God Almighty Says:
”O my servants! I have forbidden injustice to Myself and have forbidden that for you as well, then do not commit injustice to each other”. [(Muslim), Qudsi Hadith]
This verse means, that He will judg everyone correctly and no one will be wronged.
See, a person like: an atheist or Buddhist or Hindu or those who do shirk and disbelief(e.g. People of Scriptures) never heard about Islam or misunderstand it, and died in state of disbelief without knowing correctly or know a single thing about Islam and did more good deeds than bad, then this person either a girl or boy will go to Heaven. And Indeed God is the Most Just. And God Said:
”We will never punish anyone until We have sent a Messenger to them” ([17:15]).
God Almighty Says:
”We do not punish until We send a Messenger” (Quran 17:15).
Ibn Katheer (may God have mercy on him) said:
”A person who has never heard of Islam or the Prophet SAWS (peace and blessings of God be upon him), and who has never heard the message in its correct and true form, will not be punished by God if he dies in a state of disbelief. If it were asked what his fate will be, the answer will be that God will test him on the Day of Resurrection: if he obeys, he will enter Paradise and if he disobeys he will enter Hell”
God Almighty Said:
”Verily, those who disbelieve and did wrong [by concealing the truth about Prophet Muhammad and his message of true Islamic Monotheism written in the Torah and the Gospel with them]; God will not forgive them, nor will He guide them to any way”
And God Almighty also Says:
”Surely, those who disbelieved in Our Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.), We shall burn them in Fire. As often as their skins are roasted through, We shall change them for other skins that they may taste the punishment”
Except the way of Hell, to dwell therein forever”[al-Jaathiyah, 45:35]
And secondly, it wouldn’t make sense if a person who disbelief in God and in His messengers and the Book and whatever He tells, and die in state of disbelief would renter Paradise? He will never treat anyone unfairly. Because only those who believe in Him and in whatever He tells, and die in state of belief such as Muslims, will enter Heaven, and those before Muslims such as “People of Book” if God wish will enter Heaven, because God Almighty Says:
Surely, those who disbelieved in Our Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.), We shall burn them in Fire. As often as their skins are roasted through, We shall change them for other skins that they may taste the punishment”
Except the way of Hell, to dwell therein forever”[al-Jaathiyah, 45:35]
Because He doesn’t like the disbelievers and sinners.
And He also Says:
“Surely, those who believe, those who are the Jews and the Sabians and the Christians – whosoever believed in God and the Last Day, and worked righteousness, on them shall be no fear, nor shall they grieve.” [al-Maa’idah 5:69]
“Verily, those who believe and those who are Jews and Christians, and Sabians, whoever believes in God and the Last Day and do righteous good deeds shall have their reward with their Lord, on them shall be no fear, nor shall they grieve.” [al-Baqarah 2:62]
The Messenger of God, peace and blessings be upon him, said:
”He who died knowing that there is no God but God will enter Paradise”
And do you like someone to disbelief in you? Of course not, so God does not like someone who disbelieve intentionally in Him and in whatever He Tells and He likes only the good believers.
And God is All-Seer of His slaves.
And God knows best.