[21].Al-Anbiya' [The Prophets]: Ayat 39-41

Ayat 39. If only those who disbelieved knew (the time) when they will not be able to ward off the Fire from their faces, nor from their backs; and they will not be helped.

Ayat 40. Nay, it (the Fire or the Day of Resurrection) will come upon them all of a sudden and will perplex them, and they will have no power to avert it, nor will they get respite. 

Ayat 41. Indeed (many) Messengers were mocked before you (O Muhammad [SAWW](PBUH), but the scoffers were surrounded by that, whereat they used to mock.

[Tafseer] If only the Unbelievers knew (the time) when they will not be able to ward off the fire from their faces, nor yet from their backs, and (when) no help can reach them! They would not be so unreasonable if they only realised the terrible future for them! The Fire will envelop them on all sides, and no help will then be possible. Is it not best for them now to turn and repent? The Punishment may come too suddenly, as is said in the next verse. Nay, it may come to them all of a sudden and confound them: no power will they have then to avert it, nor will they (then) get respite. Mocked were (many) messenger before thee; But their scoffers were hemmed in by the thing that they mocked. The same verse occurs at Surah Al-Anam Ayat 34. "Verily, (many) Messengers were denied before you (O Muhammad  [SAWW](PBUH), but with patience they bore the denial, and they were hurt, till Our Help reached them, and none can alter the Words (Decisions) of Allâh. Surely there has reached you the information (news) about the Messengers (before you).  

Hazrat Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah [SAWW](PBUH) said, "Everyone of my Ummah will enter Jannah except those who refuse". He was asked: "Who will refuse?'' He (PBUH) said, "Whoever obeys me, shall enter Jannah, and whosoever disobeys me, refuses to (enter Jannah)".

[Al-Bukhari Book 9, Chapter 92, Hadith # 384]

Lesson : as mentioned above in Surah Al-Anbiya' Ayat 41."but the scoffers were surrounded by that, whereat they used to mock" The word Ummah here stands for the community which accepted the invitation of the Prophet (PBUH), that is to say, the people who on his call embraced Islam. Out of this community who follow him faithfully will go to Jannah.