This may serve as the reason for the preceding prohibition...

This may serve as the reason for the preceding prohibition, "And let not those grieve you", or it may be treated as the reason of the preceding reason, "surely they can do no harm "—because it has a general import which may explain the reason of a particular reason. The meaning thus will be as follows: We have said that those who fall into disbelief hastily can do no harm to Allah, because no unbeliever can do any harm to Him. QUR'AN: And let not those who disbelieve think. . .

a disgraceful chastisement: After putting the Prophet's mind at rest regarding the unbelievers' falling into unbelief hastily, and explaining that in all this they are in fact subsequent to Allah's plan, Who is driving them to a stage where they shall have no share in the hereafter, Allah now turns to the unbelievers themselves.

He tells them that they should not be happy with the respite granted to them by Allah, because Allah through this respite is giving them a chance to pile up sins over sins, and behind it all there is a disgraceful chastisement waiting for them — there is nothing for them except shame and ignominy in the hereafter. This all is based on the divinely established system of completion that every creature should bring his potentials to fruition, according to his own choice.

QUR'AN: On no account will Allah leave the believers then you shall have a great reward: Now the Speaker turns to the believers. He explains to them that the system of test and trial covers them too, in order that they too should reach the stage of completion; and the purified believer may be distinguished from the impure, and evil and wicked ones may be separated from the good and virtuous ones. The next sentence aims at removing a possible misconception.

It could be assumed that there was another way of distinguishing evil from good; that is, Allah could let the believers know who was wicked and evil so that they could avoid him. Thus, they could easily be saved all these troubles and turmoil which they had to suffer because of their commingling with hypocrites and those whose hearts were diseased.

Allah erases such erroneous impression by pointing out that He has exclusively reserved the knowledge of unseen to Himself, He does not reveal it to anyone except to some chosen messengers whom He might acquaint with it. This is the import of the sentence, "nor is Allah going to make you acquainted with the unseen, but, Allah chooses of His messengers whom He pleases".