Tafsir of Al-Mulk 67:18

"And indeed, those before them denied"—that is, those from the nations of the past who were before the disbelievers of Mecca, such as the people of Noah, ‘Ad, and their ilk. The shift to the third person [from the second person used previously] is to manifest turning away from them.

"Then how was My rejection?"—that is, My denial of them through the sending down of punishment. It was of the utmost terror and horror. This is the subject of the emphatic oath, not merely their denial. The discussion regarding "rejection" (nakīr) is like the discussion regarding "warner" (nadhīr). In this discourse, there is an exaggeration in consoling the Messenger of Allah (may Allah exalt him and grant him peace) and an intensification of the threat to his people, the extent of which is not hidden.