Exclusionary practices in dominant market-based systems are recognized as contributing to global health inequities. Undocumented immigrants are particularly vulnerable to unequal access to healthcare. Humanitarian NGOs strive to respond meaningfully to these health inequities among migrants and undocumented immigrants. Few studies describe the work of humanitarian NGOs that advocate for the right to health of undocumented immigrants in high-income countries. This paper discusses immigration, health, and human rights while examining solidarity, spirituality, and advocacy using a U.S.-based example of medical humanitarianism: the \'Our Lady of Guadalupe Free Clinic.\' In 2011, the Free Clinic began in the basement of a Catholic parish in Minnesota in response to the lack of access to medical services for undocumented immigrants. Run by a local grassroots organization, it is held every six weeks and offers free primary healthcare to Latino immigrants and the uninsured. In this article, we examine the tricky relationship between humanitarianism and human rights in the U.S. Using ethnography, we draw on participant observation and interviews with 30 clinic volunteers, including health professionals, administrators, language interpreters, and spiritual leaders. The study was conducted September 2012-December 2013 in southern Minnesota. We examine how notions of solidarity, spirituality, and advocacy structure faith-based medical humanitarianism in the U.S. and explore the underlying tensions between the humanitarian mandate, spiritual teachings (social justice, solidarity), and political advocacy. Examining a moment of \"crisis\" in the Clinic, our study shows that volunteers experience the alliance between spirituality and advocacy with uneasiness. While a spiritual calling may initially motivate volunteers to serve, an embrace of human rights advocacy is important in a sustained effort to provide humanitarian medical care to individuals who fall outside of the political and moral community in the U.S.