Has he invented about Allāh a lie or is there in him madness?" Rather, they who do not believe in the Hereafter will be in the punishment and [are in] extreme error.
(Those who disbelieve say: "Shall we direct you to a man who will tell you (that) when you have become fully disintegrated into dust with full dispersion...") means, when your bodies have disintegrated into the earth and dispersed without a trace,
إِنَّكُمْ
(then you) means, after this has happened,
لَفِى خَلْقٍ جَدِيدٍ
(will be created (again) anew) means, you will be restored to life and will receive provision anew. `By telling us this, he is doing one of two things: either he is deliberately fabricating something and saying that Allah has revealed it to him, or he is not doing it deliberately, but he is deluded as crazy and insane people are.' They said:
They say: أَفْتَرَىٰ عَلَى اللَّـهِ كَذِبًا أَم بِهِ جِنَّةٌ (Has he forged a lie against Allah, or is there a madness in him?"- 34:8). The drive of their remark is that the idea that all totally disintegrated particles of the body will stand reassembled together, turn into the human body once again and be alive as well, is so unreasonable, to start with, that there remains practically no question of entertaining or accepting it. Therefore, this claim of the Holy Prophet ﷺ is either a deliberate attempt to fabricate a lie against Allah Ta’ ala, or the person who says so is insane and no premise of his statement is correct.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 6–8
The addressees of the Quran were not disbelievers in the Day of Judgement. It was just that they did not believe that Judgement Day would bring them humiliation and punishment. The world they lived in appeared safe to them and they could not, therefore, understand how they would be in jeopardy on reaching the world Hereafter. This truth about life and the Universe is found in all divine scriptures. It is the Quran’s mission to reveal this reality in its pure and unadulterated form. Now, those who take a stand in opposition to this mission are indulging in the most improper bravado. Before God, they will be treated as deserving of the severest punishment.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.