This article explores the implications of attaching military chaplains and similar religious personnel to State and non-State fighting forces, and what this means for international humanitarian law (IHL). IHL assigns religious personnel a non-combatant humanitarian function equivalent to medical personnel, stipulating that they should perform exclusively religious duties. This underestimates the scope of “religious” activity, however, particularly the moral dimension of their ministry and the force-multiplying and restraining effects that this has on combatant behaviour. As representatives of non-State institutions embedded within military structures, many religious personnel also enjoy a unique degree of access to – and separation from – the chain of command, and can leverage this autonomy to influence the conduct of hostilities. The more that religious personnel are invested in the achievement of a fighting force's military objectives and are involved in its military operations, the likelier it is that they will test the parameters of their humanitarian function, and the protections they enjoy, under IHL. Moreover, some clerics associated with fighting forces do not aspire to non-combatant or exclusively humanitarian status, and should not be considered religious personnel. It is in the midst of armed conflict that religious personnel are most needed, however, and the tensions and ambiguities between their religious and military support functions are integral to their cross-cutting role. The contribution that religious personnel can make to humanizing war, and socializing IHL or corresponding religious principles, depends on them being present to support combatants and not confining themselves to a separate, but less effectual, humanitarian space. Criteria for their humanitarian exclusivity, attachment to fighting forces and protections under IHL therefore require some clarification.