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:

If the Qur'an had been revealed in the language of the non-Arabs, they would have said, because of their stubbornness:

meaning, why is it not revealed in detail in Arabic? And by way of denunciation they would have said, "What! not in Arabic nor from an Arab? - i.e., how can foreign words be revealed to an Arab who does not understand them? This interpretation was reported from Ibn 'Abbas, Mujahid, 'Ikrimah, Sa'id bin Jubayr, As-Suddi and others.

means, 'say, O Muhammad: this Qur'an, for the one who believes in it, is guidance for his heart and a cure for the doubts and confusion that exists in people's hearts.'

means, they do not understand what is in it.

{and it is blindness for them.} means they are not guided to the explanations contained therein. This is like the Ayah:

Mujahid said, "Far away from their hearts." Ibn Jarir said, "It is as if the one who is addressing them is calling to them from a distant place, and they cannot understand what he is saying." This is like the Ayah:

Taking Musa as an Example

means, they disbelieved in him and did not show him any respect.

i.e., to delay the Reckoning until the Day of Resurrection,

means, the punishment would have been hastened for them. But they have an appointed time, beyond which they will find no escape.

{But truly, they are in grave doubt thereto.} means, their words of disbelief are not due to any wisdom or insight on their part; rather they spoke without any attempt at examining it thoroughly. This was also the interpretation of Ibn Jarir, and it is a plausible interpretation. And Allah knows best.