This article attempts to bridge the gap between social and religious explanations for Islamist radicalisation in the West by understanding the role of religion through the under-utilised perspective of sociology of religious emotions and Wiktorowicz’s concept of cognitive openness. The article draws on interviews with 23 different actors with first-hand knowledge of Islamist radicalisation, and analyses five in-depth interviews with former so-called radicals, four of whom were converts to Islam. The analysis thus has a special focus on the narratives and experiences of converts to radical Islamist worldviews. The radicalisation process of the formers was characterised by an interplay between context specific experiences and individual religiosity. There are social causes for seeking religion as it can provide an emotional meaningfulness in a state of cognitive openness connected to personal family social background, which can stretch over a long period. However, the interviews also show that religiosity affects the social: the religious emotions within radical Islamist groups create a tight-knit community of self-perceived righteous believers, tied to an emotional experience of empowerment that amplifies their radicalisation. The article concludes that the primary role of religion is to structure and direct the emotions from which so-called radical Islamists think and act within religious frameworks.