أَأُنزِلَ عَلَيْهِ الذِّكْرُ مِن بَيْنِنَا
...
Has the Reminder been sent down to him (alone) from among us?
They thought it unlikely that he would be singled out from among them to receive the Qur'an.
This is like the Ayat:
لَوْلاَ نُزِّلَ هَـذَا الْقُرْءَانُ عَلَى رَجُلٍ مِّنَ الْقَرْيَتَيْنِ عَظِيمٍ
And they say: "Why is not this Qur'an sent down to some great man of the two towns." (43:31)
Allah said:
أَهُمْ يَقْسِمُونَ رَحْمَةَ رَبِّكَ نَحْنُ قَسَمْنَا بَيْنَهُمْ مَّعِيشَتَهُمْ فِى الْحَيَوةِ الدُّنْيَا وَرَفَعْنَا بَعْضَهُمْ فَوْقَ بَعْضٍ دَرَجَـتٍ
Is it they who would portion out the mercy of your Lord It is We Who portion out between them their livelihood in this world, and We raised some of them above others in ranks. (43:32)
When they said this, it indicated their ignorance and lack of understanding since they thought it was unlikely that the Qur'an would be revealed to the Messenger and not to somebody else.
...
بَلْ هُمْ فِي شَكٍّ مِّن ذِكْرِي
...
Nay, but they are in doubt about My Reminder!
...
بَلْ لَمَّا يَذُوقُوا عَذَابِ
Nay, but they have not tasted (My) torment!
means, they say this because they have not yet tasted the punishment and vengeance of Allah. But they will come to know the consequences of what they say and what they rejected on the Day when they are herded into the fire of Hell.
Then Allah points out that He is the One Who is in control of His Creation and Who does whatever He wills, Who gives whatever He wants to whomever He wants, and honors whomever He wants and humiliates whomever He wants, and guides whomever He wants and misguides whomever He wants, and sends the Ruh (Jibril) by His command upon whomsoever He wants among His servants, and seals the hearts of whomever He wants, so no one can guide him apart from Allah. His servants do not possess any power and have no control over His dominion, not even a speck of dust's weight; they do not possess even a thin membrane over a date stone.
Allah says, denouncing them:
أَمْ عِندَهُمْ خَزَايِنُ رَحْمَةِ رَبِّكَ الْعَزِيزِ الْوَهَّابِ