"In Western prisons, inmates’ religious conversions are a social fact in which fear plays a predominant role. Based on a qualitative and longitudinal survey conducted over two years in a French prison, this article aims to show how religious fears emerge in the consciousness of non-religious prisoners. Phenomenologically, the empirical data show that these fears arise because of an incapacitating state of anxiety. Over time, life in prison affects the social identity of individuals, their structuring markers and their biographical continuity. But the social mechanisms of anxiety are not those of fear. It is diffuse anxiety that allows the appearance of a new and identified fear. This enigma is interesting for the sociology of knowledge and therefore of socialisation: individuals are not afraid of what they do not know. The survey highlights that the emergence of these fears of God, sin, hell and so on reveals a process of regression of the habitus of the inmates to their primary socialisations. As irrational as they may seem for the frightened themselves, these fears show a structuring potential by interpreting and organising the disconcerting distress of which they are the product. Fear – religious or not – integrates and synthesises anxiety. From that moment, the appearance of fear augurs a gain of psychic, intellectual and finally practical mastery of a situation without which it would have remained unmanageable. This is how intramural trajectories become conversion paths."