﴿ يُدْخِلُ مَن يَشَآءُ فِى رَحْمَتِهِۦ ۚ وَٱلظَّٰلِمِينَ أَعَدَّ لَهُمْ عَذَابًا أَلِيمًۢا ﴾

“He admits unto His grace everyone who wills [to be admitted]; but as for the evildoers - for them has He readied grievous suffering [in the life to come].”

He admits whomever He will into His mercy His Paradise namely the believers; and as for the evildoers al-zālimīna is in the accusative because of an implied verb that is to say a‘adda ‘He has prepared’ as explained by what follows He has prepared for them a painful chastisement — these are the disbelievers.