Tafsir of Al-Isra 17:86

{لنذهبن} (We would surely take it away): This is the response to an omitted oath, acting in place of the conditional consequence (jaza’ al-shart). The lam prefixed to in serves to introduce the oath (muwatti’ah lil-qasam).

The meaning is: If We willed, We would take the Qur’an away and erase it from hearts and codices, leaving no trace of it. You would remain as you were, knowing nothing of the Book.

{ثم لا تجد لك به علينا وكيلا} (Then you would not find for yourself any advocate against Us regarding it): After We have taken it away, you would find no one to act as your advocate to Us for its recovery and restoration, preserved and recorded.

{إلا رحمة من ربك} (Except for a mercy from your Lord): This means: unless your Lord has mercy on you and restores it to you. It is as if His mercy acts as your advocate in restoring it. Alternatively, it may be an istithna’ munqati’ (disjunctive exception), meaning: "But a mercy from your Lord has left it un-removed."

This is a favor from Allah, the Exalted, regarding the preservation of the Qur’an, following the even greater favor of its revelation and the enabling of its memorization. Every possessor of knowledge must not be heedless of these two favors and must fulfill the duty of gratitude for them: the favor of Allah granting him the preservation of knowledge and its firm rooting in his heart, and the favor of its continued existence in its preserved state.

It is narrated from Ibn Mas‘ud: "The first thing you will lose of your religion is trustworthiness, and the last thing you will lose is prayer. There will be people who pray, yet they have no religion. And this Qur’an—you will wake up one day and there will be nothing left of it among you." A man asked, "How can that be, when we have established it in our hearts and written it in our codices, and we teach it to our children, and our children teach it to their children?" He replied, "It will be taken away in a single night; people will wake up destitute of it. The codices will be lifted, and what is in the hearts will be extracted."

{قل لئن اجتمعت الإنس والجن على أن يأتوا بمثل هذا القرءان لا يأتون بمثله ولو كان بعضهم لبعض ظهيرا}

"Say: 'If mankind and the jinn gathered to produce the like of this Qur’an, they would not produce the like of it, even if they were to each other assistants.'"