لِّيَسْأَلَ الصَّادِقِينَ عَن صِدْقِهِمْ ۚ وَأَعَدَّ لِلْكَافِرِينَ عَذَابًا أَلِيمًا (8)
(33:8) so that (their Lord) may question the truthful about their truthfulness. *16 As for the unbelievers, He has kept a painful chastisement in store for them. *17
*16) That is, "Allah has not just taken the covenant but will also question as to how far it has been kept. Then only those who will have kept Allah's Covenant faithfully, will be declared to be the truthful. "
*17) To understand the theme of vv. 1-8 well these should be read together with vv. 36-41 of this Surah.