• If you were to ask: Isn't it reasonably possible for them to believe in the verse that states they will not believe? So isn't this a contradiction in terms?

You would be told: They are not being ordered to believe in every detail of the Qur'an, that it should be impossible. Also, the verb "(who) reject faith (lit. disbelieved) (kafarû)" being in the perfect tense infers that they chose unbelief after the truth had become clear, so the warning was futile. [As the word sawâ' un - it is the same - indicates.]¹

Now for "the same (sawâ'un)-," it is a figure of speech meaning: whether it is futile or has the desired result, it makes no difference whether or not you warn them. That is, there's no reason for you either to warn them or to not warn them.

"To (lit. on) them ('alayhim):" in this is a sign that they shall remain on the earth but not raise their heads and heed their leaders. It also implies: "But it is not the same for you, [O Muhammad!, whether you warn them or not], for it is meritorious for you to convey the message; indeed, ' The Messenger's duty is but to proclaim [the Message]."'(5:99)

"Whether you warn them or do not warn them (a' andhartahum am lam tundhirhum):" the "whether (a' - hamza)" and "or (am)" here are like letters signifying "the same (sawâ'un)" and corroborate "the same" in the first [phrase of the verse]. Or [their purpose] is to direct attention to the two meanings of it mentioned above.

• If you were to ask: Why is this 'sameness' expressed in interrogative form (sûrat al-istifhâm)? [That is, the "a' " of "a andhartahum."]

You would be told: If you want to alert the person you are addressing subtly yet convincingly to the fact that what he is doing is futile, you have to question him so as to draw his attention to it. [Then] he can switch mentally from the act to its result and feel certain. There is also a connection between the interrogative form and the sameness, because the question comprises it. For as far as the questioner knows, it is the same whether or not the thing about which he is asking exists. Moreover, very often the answer also implies this sameness.

------------------------------------------------------

¹  Nursi, İsârâtû' 1-İ'caz [Abdûlmecid], 79