Allah, the Exalted, informs that He is able to make all of mankind one nation upon belief, or disbelief. This is just as He said,
وَلَوْ شَآءَ رَبُّكَ لآمَنَ مَن فِى الاٌّرْضِ كُلُّهُمْ جَمِيعًا
(And had your Lord willed, those on earth would have believed, all of them together.) 10:99 Allah goes on to say,
وَلاَ يَزَالُونَ مُخْتَلِفِينَإِلاَّ مَن رَّحِمَ رَبُّكَ
(but they will not cease to disagree. Except him on whom your Lord has bestowed His mercy,) This means that people will always differ in religions, creeds, beliefs, opinions and sects. Concerning Allah's statement,
إِلاَّ مَن رَّحِمَ رَبُّكَ
(Except him on whom your Lord has bestowed His mercy,) This means that those who have received the mercy of Allah by following the Messengers are excluded from this. They are those who adhere to what they are commanded in the religion by the Messengers of Allah ﷺ. That has always been their characteristic until the coming of the finality of the Prophets and Messengers (Muhammad ﷺ). Those who received Allah's mercy are those who followed him, believed in him and supported him. Therefore, they succeeded by achieving happiness in this life and the Hereafter. They are the Saved Sect mentioned in the Hadith recorded in the Musnad and Sunan collections of Hadith. The routes of transmission of this Hadith all strengthen each other (in authenticity). In these narrations the Prophet said,
«إِنَّ الْيَهُودَ افْتَرَقَتْ عَلَى إِحْدَى وَسَبْعِينَ فِرْقَةً، وَإِنَّ النَّصَارَى افْتَرَقَتْ عَلَى اثْنَتَيْنِ وَسَبْعِينَ فِرْقَةً، وَسَتَفْتَرِقُ هَذِهِ الْأُمَّةُ عَلَى ثَلَاثٍ وَسَبْعِينَ فِرْقَةً، كُلُّهَا فِي النَّارِ إِلَّا فِرْقَةً وَاحِدَة»
(Verily, the Jews split into seventy-one sects, and the Christians split into seventy-two sects, and this nation (of Muslims) will split into seventy-three sects. All of them will be in the Fire except one sect.) They (the Companions) said, "Who are they (the Saved Sect) O Messenger of Allah" He said,
«مَا أَنَا عَلَيْهِ وَأَصْحَابِي»
(The sect that is upon what my Companions and I are upon.) Al-Hakim recorded this narration in his Mustadrak with this additional wording. Concerning Allah's statement,
وَتَمَّتْ كَلِمَةُ رَبِّكَ لاّمْلاّنَّ جَهَنَّمَ مِنَ الْجِنَّةِ وَالنَّاسِ أَجْمَعِينَ
(And the Word of your Lord has been fulfilled (His saying): "Surely, I shall fill Hell with Jinn and men all together.") Allah, the Exalted, informs that He precedes everything in His preordainment and decree, by His perfect knowledge and penetrating wisdom. The result of this decree is that from those whom He has created, some deserve the Paradise and some deserve the Hell Fire. From this decree is that He will fill the Hellfire with both mankind and Jinns. His is the profound evidence and the perfect wisdom. In the Two Sahihs it is recorded that Abu Hurayrah said that the Messenger of Allah ﷺ said,
«اخْتَصَمَتِ الْجَنَّةُ وَالنَّارُ فَقَالَتِ الْجَنَّةُ: مَا لِي لَا يَدْخُلُنِي إِلَّا ضُعَفَاءُ النَّاسِ وَسَقَطُهُمْ وَقَالَتِ النَّارُ: أُوثِرْتُ بِالْمُتَكَبِّرِينَ وَالْمُتَجَبِّرِينَ. فَقَالَ اللهُ عَزَّ وَجَلَّ لِلْجَنَّةِ: أَنْتِ رَحْمَتِي أَرْحَمُ بِكِ مَنْ أَشَاءُ، وَقَالَ لِلنَّارِ: أَنْتِ عَذَابِي أَنْتَقِمُ بِكِ مِمَّنْ أَشَاءُ، وَلِكُلِّ وَاحِدَةٍ مِنْكُمَا مِلْؤُهَا، فَأَمَّا الْجَنَّةُ فَلَا يَزَالُ فِيهَا فَضْلٌ، حَتَّى يُنْشِىءَ اللهُ لَهَا خَلْقًا يُسْكِنُ فَضْلَ الْجَنَّةِ، وَأَمَّا النَّارُ فَلَا تَزَالُ تَقُولُ: هَلْ مِنْ مَزِيدٍ حَتَّى يَضَعَ عَلَيْهَا رَبُّ الْعِزَّةِ قَدَمَهُ فَتَقُولُ: قَطْ قَطْ وَعِزَّتِك»
(Paradise and the Hellfire debated. Paradise said, `None will enter me except the weak and despised of the people.' The Hell- fire said, `I have inherited the haughty and the arrogant people.' Then Allah said to the Paradise, `You are My mercy and I grant mercy with you to whoever I wish.' Then He said to the Hellfire, `You are My torment and I take vengeance with you upon whoever I wish. I will fill each one of you.' However, the Paradise will always have more bounties, to such an extent that Allah will create more creatures to dwell in it and enjoy its extra bounties. The Hellfire will continue saying, `Are there anymore (to enter me),' until the Lord of might places His Foot over it. Then it (Hell) will say, "Enough, enough, by Your might!")