Has the Qur'an been revealed to all mankind or only to the people of Arabia?
Christians and other non-Muslims often say that the Qur'an was revealed only for the Arab nation, that Muhammad was their prophet and not just our prophet. So I will not obey the Quran.
وَمَا أَرْسَلْنَاكَ إِلَّا كَافَّةً لِلنَّاسِ بَشِيرًا وَنَذِيرًا وَلَٰكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ
We have sent you ˹O Prophet˺ only as a deliverer of good news and a warner to all of humanity, but most people do not know. 
وَمَا أَرْسَلْنَاكَ إِلَّا رَحْمَةً لِلْعَالَمِينَ
We have sent you ˹O Prophet˺ only as amercy for the whole world. 
بِسْمِ اللَّهِ الرَّحْمَٰنِ الرَّحِيمِ تَبَارَكَ الَّذِي نَزَّلَ الْفُرْقَانَ عَلَىٰ عَبْدِهِ لِيَكُونَ لِلْعَالَمِينَ نَذِيرًا
Blessed is the One Who sent down the Standard to His servant, so that he may be a warner to the whole world. 
قُلْ يَا أَيُّهَا النَّاسُ إِنِّي رَسُولُ اللَّهِ إِلَيْكُمْ جَمِيعًا الَّذِي لَهُ مُلْكُ السَّمَاوَاتِ وَالْأَرْضِ ۖ لَا إِلَٰهَ إِلَّا هُوَ يُحْيِي وَيُمِيتُ ۖ فَآمِنُوا بِاللَّهِ وَرَسُولِهِ النَّبِيِّ الْأُمِّيِّ الَّذِي يُؤْمِنُ بِاللَّهِ وَكَلِمَاتِهِ وَاتَّبِعُوهُ لَعَلَّكُمْ تَهْتَدُونَ
Say, ˹O Prophet,˺ “O humanity! I am Allah’s Messenger to you all. To Him ˹alone˺ belongs the kingdom of the heavens and the earth. There is no god ˹worthy of worship˺ except Him. He gives life and causes death.” So believe in Allah and His Messenger, the unlettered Prophet, who believes in Allah and His revelations. And follow him, so you may be ˹rightly˺
قُلْ إِنْ كُنْتُمْ تُحِبُّونَ اللَّهَ فَاتَّبِعُونِي يُحْبِبْكُمُ اللَّهُ وَيَغْفِرْ لَكُمْ ذُنُوبَكُمْ ۗ وَاللَّهُ غَفُورٌ رَحِيمٌ
Say, ˹O Prophet,˺ “If you ˹sincerely˺ love Allah, then follow me; Allah will love you and forgive your sins. For Allah is All-Forgiving, Most Merciful.” 
The Arabic word كَآفَّةً used here means "The whole". The Arabic word لِّلنَّاسِ used here means "For mankind". The Arabic word بَشِيرًا used here means "As a giver of good news".
كَافَّةً لِلنَّاسِ بَشِيرًا = The Wohle Mankind As a giver of Good News.
> Surah 34 verse 28 Tafsir Ibn Kathir :-
The Prophet was sent to all of Mankind
Allah says to His servant and Messenger Muhammad:
وَمَا أَرْسَلْنَاكَ إِلاَّ كَافَّةً لِّلنَّاسِ بَشِيرًا وَنَذِيرًا
And We have not sent you except as a giver of glad tidings and a warner to all mankind,
i.e., to all of creation among those who are accountable for their deeds.
This is like the Ayah:
قُلْ يَأَيُّهَا النَّاسُ إِنِّى رَسُولُ اللَّهِ إِلَيْكُمْ جَمِيعًا
Say:"O mankind! Verily, I am sent to you all as the Messenger of Allah." (7:158)
تَبَارَكَ الَّذِى نَزَّلَ الْفُرْقَانَ عَلَى عَبْدِهِ لِيَكُونَ لِلْعَـلَمِينَ نَذِيراً
Blessed be He Who sent down the Criterion to His servant that he may be a warner to the all creatures. (25:1)
a giver of glad tidings and a warner means,
to bring the glad tidings of Paradise to those who obey you and to warn of the fire of Hell to those who disobey you.
This is like the Ayat:
وَلَكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ
and most of men know not.
وَمَأ أَكْثَرُ النَّاسِ وَلَوْ حَرَصْتَ بِمُوْمِنِينَ
And most of mankind will not believe even if you desire it eagerly. (12:103)
وَإِن تُطِعْ أَكْثَرَ مَن فِى الاٌّرْضِ يُضِلُّوكَ عَن سَبِيلِ اللَّهِ
And if you obey most of those on the earth, they will mislead you far away from Allah's path. (6:116)
Muhammad bin Ka`b said concerning the Ayah:
وَمَا أَرْسَلْنَاكَ إِلاَّ كَافَّةً لِّلنَّاسِ
(And We have not sent you except to all mankind) meaning,
to all the people.
Qatadah said concerning this Ayah,
"Allah, may He be exalted, sent Muhammad to both the Arabs and the non-Arabs, so the most honored of them with Allah is the one who is most obedient to Allah."
In the Two Sahihs it was reported that Jabir, may Allah be pleased with him said,
"The Messenger of Allah said:
أُعْطِيتُ خَمْسًا لَمْ يُعْطَهُنَّ أَحَدٌ مِنَ الاَْنْبِيَاءِ قَبْلِي نُصِرْتُ بِالرُّعْبِ مَسِيرَةَ شَهْرٍ وَجُعِلَتْ لِيَ الاَْرْضُ مَسْجِدًا وَطَهُورًا فَأَيُّمَا رَجُلٍ مِنْ أُمَّتِي أَدْرَكَتْهُ الصَّلَةُ فَلْيُصَلِّ وَأُحِلَّتْ لِيَ الْغَنَايِمُ وَلَمْ تَحِلَّ لاَِحَدٍ قَبْلِي وَأُعْطِيتُ الشَّفَاعَةَ وَكَانَ النَّبِيُّ يُبْعَثُ إِلَى قَوْمِهِ خَاصَّةً وَبُعِثْتُ إِلَى النَّاسِ عَامَّة
I have been given five things which were not given to any of the Prophets before me.
I have been aided by fear (the distance of) a month's journey.
The entire earth has been made a Masjid and a means of purification for me, so that when the time for prayer comes, any man of my Ummah should pray.
The spoils of war have been made permissible for me, whereas they were not permitted for any before me.
I have been given the power of intercession; and
the Prophets before me were sent to their own people, but I have been sent to all of mankind."
It was also recorded in the Sahih that the Messenger of Allah said:
بُعِثْتُ إِلَى الاَْسْوَدِ وَالاَْحْمَر
I have been sent to the black and the red.
"This means to the Jinn and to mankind."
Others said that it meant the Arabs and the non-Arabs.
Both meanings are correct.
How the Disbelievers asked about when the Resurrection would happen, and the Response to Them
Allah tells us how the disbelievers believed it unlikely that the Hour would ever come to pass:
 وَيَقُولُونَ مَتَى هَذَا الْوَعْدُ إِن كُنتُمْ صَادِقِينَ
> It is clear from the verses that the Holy Qur'an has been revealed for all mankind. So the whole nation should follow this book.
 (Surah Saba Varse 28)
 (Surah Al-Anbiya Varse 107)
 (Surah Al-Furqan Varse 1)
 (Surah Al- A'raf Varse 158)
 (Surah Aal-e-Imran Varse 31)
 (Tafsir Ibn Kathir)