The Institutions of State in the Khilafah by Hizb ut-Tahrir

The Khalifah is the man who represents the Ummah in ruling, authority and in the implementation of the Divine laws (Shariah). Islam has decreed that ruling and authority belong to the Ummah. It is therefore for the Ummah to appoint an individual to administer that authority and apply the divine laws on her behalf. Allah made it obligatory upon the Ummah to execute all of the Shariah. Since the Khalifah is appointed by the Muslims, this makes him a representative of the Ummah in terms of ruling, authority and in the implementation of the Shariah rules. No-one can become a Khalifah unless the Ummah has given him the pledge of allegiance (Bayah) because ruling, authority and the implementation of the Shariah belong to the Ummah in origin. By giving Bay’ah to a man as Khalifah, the Ummah effectively appoints him as her representative. Through this Bayah the Khilafah state is delegated to him, giving him the authority (Sultan) and obliging the Ummah to obey him. .

The man who rules the Muslims does not become Khalifah unless the Bay’ah is given to him by the influential people (Ahl al-Hall Wa’l-Aqd) from amongst the Ummah, without compulsion. He must fulfil certain obligatory conditions for a Khalifah and should proceed after that in implementing the rules of the Shariah.