"So do not let one who does not believe in it divert you from it"—meaning: from believing in it—"and follow his desire, for then you would perish"—meaning: you would be destroyed.
﴿فَلَا يصدنك عَنْهَا﴾ أَيْ: عَنِ الْإِيمَانِ بِهَا ﴿مَنْ لَا يُؤمن بهَا﴾. ﴿فتردى﴾ أَي: تهْلك.