Tafsir of Al-Isra 17:86

"And if We willed, We could surely take away that which We have revealed to you"—namely, the Quran, which is a healing and a mercy for the believers, and which We have made you steadfast upon when they almost tempted you away from it, along with the other attributes implied by the context. It is expressed here via the relative pronoun (that which) to exalt its status and to describe it—through what is contained in the relative clause—as being, from the very outset, a declaration of its nature: that it is not of the category of created speech.

The first 'lam' (in la-in) is preparatory for an oath. "(And We would surely take away)" is the response to the oath, standing in place of the conditional result; it thus suffices in place of estimating a conditional result, and it is not the actual result of the condition itself because the 'lam' has entered upon it. This is evident, and due to this, it is appropriate to omit the object of the verb "willed."

By "taking it away" is meant erasing it from the codices (mushafs) and the hearts—an expression more emphatic than other verbs. What is meant by this, regarding the Quran, is—as has been said—its form, so that it would no longer exist in the inscriptions of writing or in the images retained within the retentive faculty.

"(Then you would not find for yourself, regarding it, a trustee over Us)"—that is, someone to guarantee or commit to its recovery after We have taken it away, just as a trustee commits to that which they are entrusted with, while [you] expect it to be preserved in the lines and the hearts as it was before. Thus, "trustee" is a metaphor for what has been mentioned.