And We had already given Moses the Scripture, but it came under disagreement. And if not for a word [i.e., decree] that preceded from your Lord, it would have been concluded between them. And indeed they are, concerning it [i.e., the Qur’ān], in disquieting doubt.
Allah tells us that the Qur'an is so eloquent and perfect in its wording and meanings, yet despite that the idolators do not believe in it. He tells us that their disbelief is the disbelief of stubbornness, as He says elsewhere:
(And if We had revealed it unto any of the non-Arabs, and he had recited it unto them, they would not have believed in it.) (26:198-199). If the Qur'an had been revealed in the language of the non-Arabs, they would have said, because of their stubbornness:
أَأَعْجَمِيٌّ وَعَرَبِيٌّ (Is it a non- Arabic [ book ] and an Arab [ messenger ]? - 44) The original word used for non-Arabic in the text is a'jamiyy (with hamza before the letter 'ain) which is an Arabic term for ` ineloquent speech', while ` ajam' (without hamzah in its beginning) means any nation other than Arabs. Therefore ` ajamiyy is a person who is non-Arab, even though he may speak eloquently, and a'jamiyy is a person who cannot speak eloquently (Qurtubi). The use of the word أَعْجَمِيٌّ ajamiyy in this verse means that if We had revealed the Qur'an in any language other than Arabic, then the Quraysh who are the first addressees of the Qur'an would have complained that they did not understand this Book and would have said wonderingly that the prophet is an ` Arab, but the book is ajamiyy which is ineloquent.
When the Quran was revealed in the Arabic language, opponents of Islam said that this was the Prophet Muhammad’s mother tongue, and therefore it was not difficult for him to write some sort of book in Arabic and present it as holy scripture. They further said that had he really been a prophet, he would have delivered discourses in a strange language with the help of God. Non-serious and insincere people alone indulge in such talk, and such people can never be satisfied. For example, if it had happened that the prophet had approached the Arab people and started talking to them in Greek or Syriac or Persian language, the people would have had grounds for saying; ‘How strange is this prophet! He says that he has come to guide the people, but he speaks in a language which is not understood by his audience.’ The fact is that only those people are able to accept the Truth who are serious about it. Those who are not serious about the Truth are unable to understand even the clearest of its points. The analogy befitting them is that of a person who is called from a very far-off place. Such a person will hear some voice, but he will be unable to make out its real sense.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.
