That is because they say, "Never will the Fire touch us except for [a few] numbered days," and [because] they were deluded in their religion by what they were inventing.
English Transliteration
Thalika biannahum qaloo lan tamassana alnnaru illa ayyaman maAAdoodatin wagharrahum fee deenihim ma kanoo yaftaroona
Tafsir of Verse 24
That, because they said, 'The Fire shall not touch us, except for a number of days'; and the lies they forged has deluded them in their religion.
This because they say: "The Fire shall not touch us but for a few numbered days": For their forgeries deceive them as to their own religion.