Surah 13. Ar-Ra'd, Ayah 13

I.e., about His transcendental existence or the quality of His Being.

According to Raghib, the expression shadid al-mihal (which occurs in the Qur'an only in this one place) signifies "powerful in contriving, in a manner hidden from man, that wherein wisdom lies"; hence my rendering.

Nay, thunder itself, which may frighten you, is but a tame and beneficent force before Him, declaring His praises, like the rest of creation. THUNDER thus aptly gives the name to this Sura of contrasts, where what we may think terrible is shown to be really a submissive instrument of good in Allah's hands.

And the angels, whom we think to be beautiful creatures of power and glory nearest to Allah, yet feel reverence and awe even as they praise His holy name.

Who is puny man, to call Allah in question? Cf. some variations on this theme in the Book of Job, e.g. chapters 38 to 41.