لِيُنذِرَ مَن كَانَ حَيًّا
...
That he or it may give warning to him who is living,
meaning, so that this plain Qur'an might warn every living person on the face of the earth.
This is like the Ayat:
لااٌّنذِرَكُمْ بِهِ وَمَن بَلَغَ
that I may therewith warn you and whomsoever it may reach. (6:19)
وَمَن يَكْفُرْ بِهِ مِنَ الاٌّحْزَابِ فَالنَّارُ مَوْعِدُهُ
but those of the sects that reject it, the Fire will be their promised meeting place. (11:17)
Those who will benefit from his warning will be those whose hearts are alive and who have enlightened insight, as Qatadah said,
"Alive of heart and alive of insight."
Ad-Dahhak said,
"This means wise."
...
وَيَحِقَّ الْقَوْلُ عَلَى الْكَافِرِينَ
and that Word may be justified against the disbelievers.
means, it is a mercy to the believers and evidence against the disbelievers.