"When they saw Our might,"—meaning Our punishment in this world—"they said: We believe in Allah alone, and we have rejected what we used to associate with Him," meaning: what we used to affirm of polytheism.
﴿فَلَمَّا رَأَوْا بأسنا﴾ عذابنا فِي الدُّنْيَا ﴿قَالُوا آمَنَّا بِاللَّهِ وَحْدَهُ وَكَفَرْنَا بِمَا كُنَّا بِهِ مُشْرِكين﴾ أَي: بِمَا كُنَّا بِهِ مُصدقين من الشّرك.