"How, then, will it be when We assemble them for a Day about which there is no doubt?" meaning: no uncertainty regarding it.

Muhammad said: The meaning—and Allah knows best—is: How will their state be on that Day? This is a form of conciseness. "And every soul will be paid in full for what it has earned." As for the believer, he will be paid in full for his good deeds in the Hereafter; and as for the disbeliever, he is recompensed for them in this world, and he shall have the punishment of the Fire in the Hereafter. [Verse 26]