Isaac," meaning: upon Abraham and Isaac. "And among their descendants," meaning: the descendants of Abraham and Isaac, "is one who does good," meaning: a monotheist, that is, a believer, "and one who is clearly unjust to himself," meaning: a polytheist, "clearly" evident in his polytheism.
Exegesis of verses from 114 to 122 of Surah as-Saffat
إِسْحاَقَ﴾ يُرِيد: عَلَى إِبْرَاهِيم وَإِسْحَاق] ﴿وَمِن ذُرِّيَّتِهِمَا﴾ [يُرِيد: ذُرِّيَّة إِبْرَاهِيم وَإِسْحَاق] ﴿مُحْسِنٌ﴾ [يُرِيد: موحدًا، يَعْنِي:] مُؤمن (وَظَالِمٌ لِّنَفْسِهِ مُشْرك [﴿مُبين﴾ بَين الشّرك.
تَفْسِير الْآيَات من ١١٤ وَحَتَّى ١٢٢ من سُورَة الصافات