"And were it not for a word that preceded from your Lord," that He would not punish the disbelievers of the end of this nation except by the Blast, "it would have been binding," meaning: they would have been compelled to face the punishment for their disbelief and would have all been destroyed, due to their rejection of what came.
﴿وَلَوْلا كَلِمَةٌ سَبَقَتْ مِنْ رَبِّكَ﴾ أَلَّا يُعَذِّبَ كُفَّارَ آخِرِ هَذِهِ الْأُمَّةِ إِلَّا بِالْنَفْخَةِ ﴿لَكَانَ لِزَامًا﴾ أَيْ: لَأُلْزِمُوا عُقُوبَةَ كُفْرِهِمْ فَأُهْلِكُوا جَمِيعًا؛ لِجُحُودِهِمْ مَا جَاءَ