Tafsir of Al-Qasas 28:86

وما كنت ترجو أن يلقى إليك الكتاب

And you were not expecting that the Book would be sent down to you, `Before the revelation was sent down to you, you did not expect that revelation would be sent down to you.'

إﻻ رحمة من ربك

but it is a mercy from your Lord. means, `but revelation has been sent down to you from Allah as a mercy to you and to mankind because of you. Since Allah has granted you this great blessing,'

فﻼ تكونن ظهيرا

So never be a supporter, i.e., a helper,

للكافرين

of the disbelievers. rather, separate from them, `express your hostility towards them and oppose them.'