This essay traces Christian thinking about sacred and secular authority during the early centuries of the Roman Empire. Christian martyrdom, interpreted by apologists such as Tertullian, established a place for Christianity in Roman society and gave it authority against imperial power. From this confrontation there emerged a differentiation of religious and civil authority that provided a starting point for later constitutional ideas of separate and balanced powers and distinctions between state and civil society. A comparative perspective reminds us, however, that (...) at their beginnings, Islam and Christianity faced quite different questions about religious and political authority. (shrink)