Al-Baqarah 2:6

Verily, those who disbelieve, it is the same to them whether you warn them or do not warn them, they will not believe (6)

Allah said,

إِنَّ الَّذِينَ كَفَرُوا

(Verily, those who disbelieve) meaning, covered the truth and hid it. Since Allah has written that they would do so, it does not matter if you (O Muhammad ﷺ) warn them or not, they would still have disbelieved in what you were sent with. Similarly, Allah said,

إِنَّ الَّذِينَ حَقَّتْ عَلَيْهِمْ كَلِمَتُ رَبِّكَ لَا يُؤْمِنُونَ - وَلَوْ جَاءَتْهُمْ كُلُّ آيَةٍ حَتَّىٰ يَرَوُا الْعَذَابَ الْأَلِيمَ

(Truly, those against whom the Word (wrath) of your Lord has been justified, will not believe. Even if every sign should come to them, until they see the painful torment)(10:96-97).

About the rebellious People of the Book, Allah said,

وَلَئِنْ أَتَيْتَ الَّذِينَ أُوتُوا الْكِتَابَ بِكُلِّ آيَةٍ مَّا تَبِعُوا قِبْلَتَكَ

(And even if you were to bring to the People of the Book (Jews and Christians) all the Ayat, they would not follow your Qiblah (prayer direction))(2:145).

These Ayat indicate that whomever Allah has written to be miserable, they shall never find anyone to guide them to happiness, and whomever Allah directs to misguidance, he shall never find anyone to guide him. So do not pity them - O Muhammad ﷺ - deliver the Message to them. Certainly, whoever among them accepts the Message, then he shall gain the best rewards. As for those who turn away in rejection, do not feel sad for them or concerned about them, for

فَإِنَّمَا عَلَيْكَ الْبَلَاغُ وَعَلَيْنَا الْحِسَابُ

(Your duty is only to convey (the Message) and on Us is the reckoning)(13:40), and,

إِنَّمَآ أَنتَ نَذِيرٌ وَاللَّهُ عَلَى كُلِّ شَىْءٍ وَكِيلٌ

(But you are only a warner. And Allah is a Wakil (Disposer of affairs, Trustee, Guardian) over all things)(11:12).

Ali bin Abi Talhah reported that Ibn 'Abbas said about Allah's statement,

إِنَّ الَّذِينَ كَفَرُواْ سَوَآءٌ عَلَيْهِمْ ءَأَنذَرْتَهُمْ أَمْ لَمْ تُنذِرْهُمْ لاَ يُؤْمِنُونَ

(Verily, those who disbelieve, it is the same to them whether you (O Muhammad ﷺ) warn them or do not warn them, they will not believe)

That the Messenger of Allah ﷺ was eager for all the people to believe and follow the guidance he was sent with. Allah informed him that none would believe except for those whom He decreed happiness for in the first place, and none would stray except those who Allah has decreed to do so in the first place.