This is a paper about the role of religion in the practice of humanitarian/devel- opment work, and the role of humanitarian/development work in the practice of religion. It contends that these are overlapping and indeed mutually constitutive categories. Two arguments are developed in support of that proposition. The first main argument concerns twentieth century shifts in the Christian con- cept of mission, in particular its transformation from the salvation of souls (conver- sion) to the betterment of people’s temporal lives (social service and humanitarian action). This includes the cessation of proselytization in favour of corporeal works of mercy, as well as the secularization of missionary societies and movements into development agencies and NGO’s, many of which downplay their religious origins. Sœur Emmanuelle (the paper’s case-study) is an exemplary figure in this evolution : she both propelled and was propelled by mission’s evolution toward the secular materialist priorities around which religion, humanitarianism, and development have converged. The second main argument concerns the blurring of lines, the overlapping of identities between institutionally separate actors, and the dissolving of distinctions between the categories of religious and non-religious. That boundary is prob- lematized as Christian faith reveals itself to have been an important catalytic and federative force in development projects with Cairo’s garbage collectors, including at the level of « non-religious » actors like the World Bank and its consultant engineers. In fact, Christian connections could be most effectual when least conspicuous – and become on the contrary a barrier to cooperation when too explicit. For instance, some overtly secular but sociologically Christian associations supporting sœur Emmanuelle play their religiosity up (in private) or down (in public) depending on the audience (of donors). An overtly « religious » group (Catholic Relief Services) refused to fund Zabbaleen development projects because supporting projects spearheaded by a nun and targeting a primarily Christian group would lend itself to accusations of sectarian favouritism.