Allah says:رَّبُّكُمْ أَعْلَمُ بِكُمْYour Lord knows you best;O mankind, meaning, He knows best who among you deserves to be guided and who does not deserve to be guided.إِن يَشَأْ يَرْحَمْكُمْif He wills, He will have mercy on you,by helping you to obey Him and turn to Him.أَوْ إِن يَشَأْ يُعَذِّبْكُمْ وَمَا أَرْسَلْنَاكَor if He wills, He will punish you. And We have not sent you (- O Muhammad -).عَلَيْهِمْ وَكِيلًas a guardian over them.meaning, `We have sent you as a warner, so whoever obeys you, will enter Paradise and whoever disobeys you, will enter Hell.