Purpose of the Study: Studies show that loneliness is a major risk factor for health issues in later life. Although research suggests that religious involvement can protect against loneliness, explanations for this general pattern are underdeveloped and undertested. In this paper, we propose and test a theoretical model, which suggests that social integration and social support are key mechanisms that link religious attendance and loneliness. Design and Methods: To formally test our theoretical model, we use data from the National Social Life Health and Aging Project (2005/2006), a large national probability sample of older adults aged 57-85 years. Results: We find that religious attendance is associated with higher levels of social integration and social support and that social integration and social support are associated with lower levels of loneliness. A series of mediation tests confirm our theoretical model. Implications: Taken together, our results suggest that involvement in religious institutions may protect against loneliness in later life by integrating older adults into larger and more supportive social networks. Future research should test whether these processes are valid across theoretically relevant subgroups.