"Indeed, those who disbelieve in the Reminder," meaning: the Quran.
"And indeed, it is a mighty Book," meaning: inaccessible.
﴿إِن الَّذين كفرُوا بِالذكر﴾ يَعْنِي: الْقُرْآن.
﴿وَإِنَّهُ لَكِتَابٌ عَزِيزٌ﴾ أَي: منيع