Consequently...
Consequently, after the death/martyrdom of an Infallible Imam, his successor who has been earlier prophesized based on a divine order and through the previous Imam, will become the succeeding Imam and the leader of the Muslim community.
It was narrated from the sixth Imam, Imam Ṣādiq (PBUH) who quoted Imam Ali (PBUH) to have said: “…Know that indeed the earth cannot be void of a Divine Authority but Allah will not, later on, let His creatures see him because of their oppression, cruelty, and committing sins.
If the earth becomes void of a Divine Authority even for one moment, it collapses with all its inhabitants…” After the martyrdom of Imam Hasan al-Askari in 260 A.H., the imamate of Imam Muhammad al-Mahdi (may Allah hasten his appearance) began. Interestingly, the imamate of Imam al-Mahdi (PBUH) began with Occultation (Ghaybat) and it has continued till the present time. Meanwhile, his imamate is divided into two parts: 1.
The Minor Occultation (al-Ghaybat al-Sughra): The first 69 years of his imamate is known as the period of the Minor Occultation (al-Ghaybat al-Sughra). During this period, Imam al-Mahdi (PBUH) appointed four special representatives, one after another, as a link between himself and his Shiites. Of course, there were other representatives also but with limited jurisdiction or limited duties. The four special representatives were as follows: I) ‘Uthmān bin Sa‘īd al-‘Amri, known as Abu Amr.
II) Muhammad bin ‘Uthmān bin Sa‘īd, known as Abu Ja‘far. III) Husayn bin Rawh an-Nawbakhti, known as Abul Qāsim. IV) ‘Ali bin Muhammad Samary, known as Abul Hasan. 2. The Major Occultation (al-Ghaybat al-Kubra) : The period after 329 A.H. till the present is known as the Major Occultation (al-Ghaybat al-Kubra).
Unlike the period of Minor Occultation (Ghaybat Sughra) where the Shiites had access to communicate with their Imam through the special deputies, there was no special deputy during the Major Occultation (Ghaybat Kubra). The Imam al-Mahdi (PBUH) will appear from the Occultation whenever Allāh allows him to do so to establish the Kingdom of God on earth, and to eradicate injustice and tyranny once and for all.
Meanwhile, during the Major Occultation, those Shiites who do not have the expertise to derive the laws from the Qur’an and the Sunna of the Infallibles should follow the guidance provided by those who are experts. A person known as Ishāq Ibn ‘Ammār asked in a letter to Imam al-Mahdi (PBUH) about the guidance during the Major Occultation.