This dissertation examines how Protestant theologians in Germany and the United States navigated the transition to historicism, the unprecedented awareness of qualitative historical change that marked Western historical consciousness in the nineteenth century. It examines what I call the 'mediating historicism' of mediating theologians (the German Vermittlungstheologen and their American students) that attempted to harmonize newer conceptions of dynamic historical development with traditional Christian theology. The metaphysically idealist character of German historicist thought in the early part of the century enabled such mediations, and thus the historiography of historicism as a necessarily secularizing form of thought requires revision. However, the mediating theologians' emphasis on justifying Christianity through contemporary historical scholarship, paired with their reliance on particular assumptions of early historicist thought to resolve problems related to the historicity of Christianity, left their mediations intellectually vulnerable to changes in historical scholarship that came at the end of the century. I examine two prominent theology professors in Prussia and the United States: Friedrich August Gottreu Tholuck (1799-1877) at the University of Halle and his American student Henry Boynton Smith (1815-77) at Union Seminary in New York. Tholuck developed forms of mediating historicism as part of a larger effort to make theology a 'believing Wissenschaft' that reconciled faith with academic knowledge in a post-Enlightenment culture that he and other mediating theologians perceived as estranged from traditional Christianity. Tholuck and other mediating theologians became important contacts in the growing American engagement with German theology after 1815, due to common forms of Protestant piety and a shared opposition to perceived threats to Protestant orthodoxy. Henry Boynton Smith demonstrates how mediating theology functioned as an early site for the reception and reconfiguration of German historicist ideas in the United States. While Smith struggled to apply mediating historicism consistently to both the study of church history and the Bible, his student Charles Briggs found in mediating historicism the key for reconciling much of recent historical criticism of the Bible with his evangelical Protestantism, even as the assumptions that sustained this reconciliation became marginal ones in both Germany and the United States.