Signs of Quranic authenticity

وَعْدَ ٱللَّهِ ۖ لَا يُخْلِفُ ٱللَّهُ وَعْدَهُۥ وَلَـٰكِنَّ أَكْثَرَ ٱلنَّاسِ لَا يَعْلَمُونَ

[It is] the promise of Allāh. Allāh does not fail in His promise, but most of the people do not know.