1. 1

   The Qurʾān was revealed in the tongue of the Arabs at a time when their eloquence and rhetoric had reached their peak.

2. 2

   It challenged them to produce its like, then ten chapters, then a single chapter — despite their strong motive to answer it.

3. 3

   They turned from rhetorical response to warfare despite its hardship — a practical admission of inability to meet the easier demand.

﴿قُل لَّئِنِ اجْتَمَعَتِ الْإِنسُ وَالْجِنُّ عَلَىٰ أَن يَأْتُوا بِمِثْلِ هَٰذَا الْقُرْآنِ لَا يَأْتُونَ بِمِثْلِهِ وَلَوْ كَانَ بَعْضُهُمْ لِبَعْضٍ ظَهِيرًا﴾

The inability of the masters of the language to answer it, despite every motive, indicates it is not human speech.