Resurrection (Ba^th), Assembly (Hashr)
It is an obligation to believe that Prophet Muhammad, may Allah raise his rank, was truthful in everything he told about and conveyed from Allah. Among what the Prophet informed us about are Resurrection (Ba^th), and Assembly (Hashr).
Know that the Day of Judgement starts with Resurrection.
Resurrection (Ba^th):
It is an obligation to believe in the Resurrection (Ba^th), because Prophet Muhammad informed us about it. The Day of Judgement starts with the Resurrection of people and that is when the bodies that have decayed are returned and they come out of their graves. But there are bodies that do not decay and these are the bodies of the Prophets, the Martyrs (only the ones who died in the battle field), and some saints, the earth does not consume their bodies.
Assembly (Hashr):
That is when the angels bring together all the people from their graves to the first Mawqif** (i.e. where the people are gathered). The people on the assembly day will be gathered in three forms.
1- The first type of people will be dressed, fed, and riding. These are the righteous Muslims. The righteous Muslim is the one who performed all the obligations and refrained from all sins.
2- The second type of people will be naked and bare foot. These are the big sinners of the Muslims. A big sinner is a person who had committed an enormous sin such as drinking alcohol, or committing adultery and died without repentance.
3- The third type of people will be naked, bare foot, and dragged on their faces.
Those are the blasphemers and the non-believers.
**There are 50 Mawqifs on the Day of Judgement, and each one of them lasts one thousand of our years.
And Allah knows best.