The 'responsibility to protect' (RtoP) concept has rapidly taken a prominent place in international debates about how to ensure the protection of civilians from mass atrocities in places such as Libya, the Congo, and Darfur. This article argues that RtoP has deep roots both in Scripture and also in Christian political thought of the last two millennia. In particular, it observes that, whereas twentieth-century arguments for 'humanitarian intervention' framed the protection of strangers and foreigners as a discretionary right, RtoP echoes Ambrose of Milan and others in framing such protection as a duty. It further suggests that the emphasis of RtoP on the responsibility to work with states and to help build their capacity to protect their civilians, rather than waiting until military intervention is necessary, holds the promise of advancing the Christian principle of assisting the vulnerable while, at least in some instances, avoiding thorny and divisive debates about the justice of war.