The Terrorism Act 2000 recognises that religion may be a force for bad in its definition of ‘terrorism’: ‘The use or threat of action … designed to influence a government or to intimidate the public or a section of the public … for the purpose of advancing a political, religious or ideological cause.’ ‘Radicalisation’ is the process by which an individual becomes involved in terrorism, and it is the statutory Prevent Duty, introduced under the Counter-Terrorism and Security Act 2015, that is the UK's current means of tackling radicalisation. Under the duty, specified authorities (such as schools, police, etc) must have due regard to preventing people from being ‘drawn into terrorism’. This article addresses an often-neglected area in the ongoing legal debate surrounding the statutory Prevent Duty: the religious dimension. It suggests that religion is at the centre of the Prevent Duty and that the formulation of the duty was stimulated by religion – namely, Islamist extremism and radicalisation. It will discuss the ways in which the Prevent Duty can be criticised; that it has an impact on human rights that it is potentially discriminatory towards Muslims; and that it does not clearly distinguish between permissible radical religion and impermissible harmful religion. The article concludes by suggesting that religion plays an important part in the understanding of extremism and radicalisation.