Who are the Ummah of Muhammad, sallallaahu ‘alayhi wa sallam? What will be their fate on the Day of Resurrection?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The expression Ummat Muhammad (nation of Muhammad), sallallaahu ‘alayhi wa sallam, is used in two different senses. It may firstly refer to everyone whom his Da‘wah (call) reached. This is supported by the Hadeeth where the Prophet, sallallaahu ‘alayhi wa sallam, swore by Allaah The Almighty that any person from the Ummah, whether Jew or Christian, who hears about him but does not believe in his message will enter Hell. [Muslim]

Thus the Ummah that is referred to in this Hadeeth is the Ummah of Da‘wah, to which the message of the Prophet, sallallaahu ‘alayhi wa sallam, was directed: i.e. all human beings and Jinn. 

On the other hand, Ummah may refer only to the Ummah of response: i.e. the people who responded to the Da‘wah of the Prophet, sallallaahu ‘alayhi wa sallam. This is supported by a Hadeeth where the Prophet, sallallaahu ‘alayhi wa sallam, said that his intercession will be for those who commit major sins in his Ummah. The belief of Ahlus-Sunnah wa'l-Jamaa‘ah (the Sunni mainstream) is that whoever dies a believer in Allaah The Almighty and in His Messenger, sallallaahu ‘alayhi wa sallam, will enter Paradise. If this person was a non-observant Muslim, he may enter Hell for a specific period according to his sins, then he will be taken out of Hell and admitted into Paradise. The Prophet, sallallaahu ‘alayhi wa sallam, stated that Allaah The Almighty will command that any person with the least amount of faith in his heart be driven out of Hell. [Al-Bukhari & Muslim].  A person who dies a disbeliever,  Allaah forbid, will reside eternally in Hell and will never leave it, though he may be apparently counted among the people of the Ummah of response. Allaah The Almighty Says in this regard (what means): {Indeed, Allaah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allaah has certainly fabricated a tremendous sin.} [Quran 4:48]

Allaah Knows best.