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.
The Saheeh International Quran Translation
ذَٰلِكَ بِأَنَّهُمْ قَالُوا۟ لَن تَمَسَّنَا ٱلنَّارُ إِلَّآ أَيَّامًا مَّعْدُودَٰتٍ وَغَرَّهُمْ فِى دِينِهِم مَّا كَانُوا۟ يَفْتَرُونَThat 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.