In Sunni Islam, a Caliph is only secondarily a religious leader, he is more of a political leader. This is why in Sunni law a Caliph does not need to be a scholar of Islam. But despite his primary role as a political and worldly leader, he still has some religious functions, such as determining for the Sunni community such as determining special days like Eid al-Fitr which is the festival that marks the end of the month of Ramadan. Although he does not have to be a scholar, he is still expected to have some acceptable level of knowledge when it comes to Islam. 

In Sunni Islam, successorship to the Prophet Muhammad (s) is not necessarily determined by the Prophet himself. It can be determined in three other ways as well: 

Through selection by a small but select group of the Prophet’s companions. 

Personal designation by the previous Caliph 

Self-declaration or self-appointment  

In Shia Islam, the Prophet’s successor can only be chosen by God. Unlike Sunni Islam, the office of the Caliphate is primarily a religious one. The Caliph must be the most knowledgeable person on earth at the time, be infallible, and be divinely inspired. The primary role of the Caliph is the religious guidance of humankind. Political office is only secondary. In Shiʿi Islam, person can still be the Prophet’s caliph or successor yet not hold political office.  

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