﴿ إِنَّا جَعَلْنَٰهُ قُرْءَٰنًا عَرَبِيًّۭا لَّعَلَّكُمْ تَعْقِلُونَ ﴾

“behold, We have caused it to be a discourse in the Arabic tongue, so that you might encompass it with your reason.”

(Lo! We have appointed it) We have spoken it, i.e. the Qur'an (a Lecture in Arabic) in the Arabic usage, hence the oath, (that haply ye may understand) so that you may know what is in the Qur'an of the lawful and unlawful, commands and prohibitions.