"When we have died and have become dust" in the form of an interrogative. "That is a far return"—they deny resurrection, meaning: it will not happen.
﴿أئذا متْنا وَكُنَّا تُرَابا﴾ على الِاسْتِفْهَام ﴿ذَلِكَ رَجْعٌ بَعِيدٌ﴾ يُنكرُونَ الْبَعْث؛ أَي: إِنَّه لَيْسَ بكائن،