"He admits whom He wills into His mercy," into His religion, Islam. "And the wrongdoers," meaning the polytheists, "He has prepared for them a painful punishment," a torment that causes pain.
Muhammad said: The accusative case (nasb) of "the wrongdoers" (al-zalimin) is based on the meaning: He admits whom He wills into His mercy, and He punishes the wrongdoers. Thus, "He has prepared for them" serves as an explanation for this implied meaning (the accusative case of the wrongdoers is based on the meaning: He admits whom He wills into His mercy, and He punishes the wrongdoers).