Given the long roots of bay‘a (pledge of allegiance) in Islamic tradition and the controversial claim by the Islamic State (IS) to be a caliphate, the application of bay‘a to the group’s leader, Abu Bakr al-Baghdadi, and his project is a contested issue among radical Islamists. Based on secondary literature and IS ideologues’ own writings, this paper analyses IS’s claims of validity in their calls for allegiance to “caliph” Abu Bakr al-Baghdadi and how radical Islamist critics of IS have responded to this. IS’s arguments resemble quite closely the theories on bay‘a that its jihadi opponents themselves claim to adhere to. Although the latter take their inspiration from early Islam and far less so from the theories that developed afterwards, it sometimes also appears as if they have idealised the caliphate so much that they find its reality as represented by IS hard to swallow.