Islamic Eschatology – Day of Judgment, Paradise & Hell Fire
Original answered on quora - 10/12/18 - https://www.quora.com/Adherents-of-non-Christian-monotheistic-religions-how-in-your-religion-can-Gods-love-towards-man-be-reconciled-with-his-holiness-and-justice-when-man-is-so-inclined-to-sin-In-Christianity-the-sacrifice-of-Jesus-on/answer/Zaid-Shaw
The Mercy and Justice of Allah ﷻ are not only reconciled but in fact His immense Grace and Mercy that we call Rahmah pervades everything including justice. This does not mean that His Grace and Mercy precludes justice, justice remains active but Mercy and Grace exceeds if and when necessary.
Firstly Allah ﷻ knows that we are created weak and prone to sinning and it is this reason why He is always encouraging and advising us to seek forgiveness and repent by giving us reminders throughout the Quran that He is Al Ghafoor (Most Forgiving), Al Ghafaar (All Forgiver), At Tawwab (The acceptor of repentance), Al Rahman (Most Gracious), Al Raheem (Most Merciful), Al Afuwwu (The One Who Pardons). Similar attributes among the 99 attributes of His mentioned in the Quran remind us about His ability to have mercy, forgiveness and pardon for each among humankind regardless of their sins provided they turn towards Him with sincere intention. (Al Quran 39:54) "SAY: [Thus speaks God:] ‘O you servants of Mine who have transgressed against your own selves! Despair not of God's mercy: behold, God forgives all sins - for, verily, He alone is much-forgiving (Al Ghafoor), a dispenser of grace! (Al Raheem)’"
To understand better the words "despair not of God’s mercy : behold God forgives all sins" we need to find out how great is His Grace and Mercy as taught us in the Quran and Prophetic teachings. There are many attributes of Allah ﷻ in the Quran but He has never used the following words when referring to any of those except for the One pertaining to His Mercy and Grace i.e. Rahmah : (Al Quran 6:54) "Your Sustainer has willed upon Himself the law of grace and mercy (Rahmah) - so that if any of you does a bad deed out of ignorance, and thereafter repents and lives righteously, He shall be [found] much-forgiving (Al Ghafoor), a dispenser of grace (Al Raheem)." (Al Quran 7:156) [God] answered: "With My chastisement do I afflict whom I will - but My grace (Rahmah) overspreads everything: and so I shall confer it on those who are conscious of Me and spend in charity, and who believe in Our messages."
As per Islamic scholars specialized in Arabic the terms such as "willed upon Himself the law of grace and mercy" (kataba ala nafsihi rahma) and "My grace overspreads everything", are not mentioned in the same manner in Quran when it comes to other attributes of Allah ﷻ including those pertaining to justice and punishment. Therefore as per Quran’s message His Mercy and Grace is unique.
Allah ﷻ encourages that we should beg forgiveness for our sins, regardless of their degree and never be hopeless of His Mercy as His Mercy is always greater than our sins. This He reminds us for the sole reason that humans have been created weak, we are prone to sinning and He is aware of that: (Al Quran 4:27–28) "And God wants to turn unto you in His mercy, whereas those who follow [only] their own lusts want you to drift far away from the right path. God wants to lighten your burdens: for man has been created weak." - In order to better understand expression of His mercy, love and justice towards humankind we need to first understand the purpose of our creation. We have to know one thing that this life has been given to us for the sake of testing us. Huankind has not been created for earth, we have been created for much better and greater than we can even imagine and that is the sole reason why we do not live on earth forever, we continue to die and be replaced: (Al Quran 67:02) "He who has created death as well as life, so that He might put you to a test [and thus show] which of you is best in conduct, and [make you realize that] He alone is almighty, truly forgiving (Al Ghafoor)." - During this test in our life we are prone to errors and sinning due to the inherent weakness in our natures as well as due to ignorance of truth, therefore Allah ﷻ has made it clear that He will continue to forgive as long as we are conscious of Him and turn towards Him: (Al Quran 4:17) "Verily, God's acceptance of repentance relates only to those who do evil out of ignorance and then repent before their time runs out: and it is they unto whom God will turn again in His mercy - for God is all-knowing, wise;"
In fact even those who committed minor sins, as opposed to major sins (Islam categorizes major & minor sins), and if they died while living in this manner, even they have hope to gain forgiveness in the hereafter despite having failed to repent: (Al Quran 4:31) "If you avoid the great sins, which you have been enjoined to shun, We shall efface your [minor] bad deeds, and shall cause you to enter an abode of glory." - There is even hope for major sinners who continuously and consciously sin and act unjust towards themselves and others ignoring the purpose of life, He reminds them too that His Mercy will always attempt to win them over: (Al Quran 18:58) "Yet, Your Sustainer is the Truly-Forgiving One (Al Ghafoor), limitless in His grace (Zhu Rahma). Were He to take them [at once] to task for whatever [wrong] they commit, He would indeed bring about their speedy punishment [then and there]: but nay, they have a time-limit beyond which they shall find no redemption." - He could’ve immediately punished us as soon as we were done sinning but He delays and continues to give us ample time so that we turn to His never ending mercy and beg His forgiveness as His purpose for us is to be good in this world and be in a better place in hereafter and not worst. But if such sinners continue to live wrong and their time runs out only then they will have to face consequences of their actions: (Al Quran 4:18) "whereas repentance shall not be accepted from those who do evil deeds until their dying hour and then say, "Behold, I now repent"; nor from those who die as deniers of the truth: it is these for whom We have readied grievous suffering."
Now we also have the belief that Prophet Muhammad ﷺ will be the primary intercessor for entire humankind on the day of judgment. He will beg for mercy of Allah ﷺ for even those who died as major sinners. The honor of greatest intercession has only been permitted for Prophet Muhammad ﷺ , and not to any other prophet whether Adam (alaihi salaam), Noah (alaihi salaam), Ibrahim (alaihi salaam), Moses (alaihi salaam) and even Jesus (alaihi salaam). This is because he is the only one whom Allah ﷻ refers to as Mercy For Entire Creation (Rahmatul lil Alameen): (Al Quran 21:107) "And [thus, O Prophet Muhammad] We have sent you as [an evidence of Our] grace (Rahmah) towards entire creation." - As result of his intercession many from among the people of hell fire including major sinners, will be released and sent to paradise.
All of this has been evidenced in other source of guidance for Muslims called hadith literature. For instance Salman (r.a) reported that Prophet Muhammad ﷺ said: “Verily, Allah created, on the same very day when He created the heavens and the earth, one hundred parts of mercy. Every part of mercy is as big as the space between the heavens and the earth and He out of this mercy endowed one part to the earth and it is because of this that the mother shows affection to her child and even the beasts and birds show kindness to one another and when there would be the Day of Resurrection, Allah would make full (use of Mercy). (reported by Sahih Muslim)  Prophet Muhammad ﷺ is an expression of the Allah’s mercy in this world as well as for the day of judgment so he will be the means of salvation for many. You can read the details regarding the day of judgment when help of all prophets will be sought but only the Final Messenger will assist: The Mercy of Allah + Prophetic Intercession = Salvation
In Islam we have details of how Allah ﷻ will deal with His entire creation especially humankind which places our hearts at ease unlike other religions which do not provide details on the details of the hereafter. A very good example is Christianity, in which its followers have to believe in "Vicarious Atonement" concept which claims "Jesus died on the cross for sins of humankind so that they may be forgiven in the hereafter because God's justice is mightier than everything and will not let anyone go unpunished." Such concepts deprive us to have a personal spiritual relationship with our Creator and keeps us ignorant of how great is His Mercy and Grace.
All Quran ayah's (verses) taken from Asad Translation.
 The Major sins consists of associating partners with God; disrespecting one's parents; consuming other's property; especially of orphans; engaging in usury (giving loans on interest); retreating when the army advances (for a battle); slandering pious and chaste women; committing crimes with a prescribed punishment (e.g. theft, fornication, adultery, murder); engaging in prohibited acts despite the Quran's or the prophetic traditions threat of a severe punishment for doing so in the hereafter; and deeds cursed by the Prophet ﷺ.
 Sahih Muslim 2753 c - Sunnah.com