The concept of RAJ'A in Shiaism is baffling. They believe that the return of Mahdi will be followed by the resurrection of those were not loyal to the Ahl e Biyat or usurped their rights in any way. Hazrat Abu Bakr, Umar and Usman, and others will be brought back to life and punished for their sins.
So much for the unity of the two sects
The word "raj'a" literally means 'to return'. In the Qur'an and narrations different terms have been used to refer to this doctrinal principle such as raj'a, karra, radd, hashr. However, raj'a is the most popular term.
This form of Arabic infinitive (masdar al-marra) for raj'a translates into 'returning once, or to return once'.
In Shi'a Beliefs
As a technical term used in Islamic discourse and science, raj'a has been coined to represent the belief that at the time of Imam al-Mahdi's reappearance, God will bring back to life a number the deceased to the world with their worldly body. Creating an opportunity for the believers to reach their ultimate perfection is mentioned among the reasons behind raj'a. According to a hadith from Imam al-Rida (a), those believers who do not achieve the blessing of martyrdom will achieve it during their raj'a.
The purpose of this returning to life is for the Shi'as to aid and help the Imam (a) in his mission and for them to witness the establishment of his righteous government. Similarly, God will also bring back to life a number of the Imam's enemies so that they may taste the punishment of this world and witness, much to their regret, the glory and power of Imam al-Mahdi (a)'s government .