This is meant to comfort the Prophet for the mockery of those among his people who opposed him, and contains a promise that Allah will help him and his religion and grant him victory over his enemies. This is like the Ayah:

{Verily, Messengers were denied before you, but with patience they bore the denial, and they were hurt ; till Our help reached than, and none can alter the Words of Allah. Surely, there has reached you the information (news) about the Messengers (before you)} [6:34].

Then Allah menitons His favor for His creatures; He protects them by night and by day, taking care of them and watching over them with His Eye that never sleeps.

{Say : "Wlw can guard and protect you in the night or it} the day from the Most Gracious?"}
means, other than the Most Gracious Himself?

means, they do not recognize the blessings and favor of Allah towards them; they turn away from His signs and blessings.

This is a rhetorical question aimed at denouncing and rebuking. The meaning is, do they have any gods who can protect them and take care of them other than Us? It is not as they imagine or as they claim. Allah says:

these gods on whom they rely instead of Allah cannot even help themselves.

{nor can they be protected from Us.} Al-‘Awfi reported from Ibn ‘Abbas, “Nor can they be guarded from Us.”
Al-Hasan Al-Basri said: “This means the victory of Islam over disbelief.” The meaning is: Do they not learn a lesson from the fact that Allah supported those (believers) against their enemies, He destroyed the disbelieving nations and the evil-doing townships, and He saved His believing servants? So Allah says:

{Is it then they who will overcome?} meaning, on the contrary, they are the ones who will be overcomed, who will be defeated, humiliated and brought low.

meaning, ‘I only convey to you the warning of Allah’s punishment and vengeance, and this is no more than that which Allah reveals to me.’ But this is of no benefit to the one whom Allah has made blind and has put a seal over his hearing and his heart. He says: