"If they turn away"—meaning: from that which the Prophet (peace and blessings of Allah be upon him) brought—"then indeed Allah is All-Knowing of the corrupters," meaning: the polytheists.
﴿فَإِن توَلّوا﴾ يَعْنِي: عَمَّا جَاءَ بِهِ النَّبِي ﷺ ﴿فَإِنَّ الله عليم بالمفسدين﴾ يَعْنِي: الْمُشْركين