Thesis advisor: Robin Fleming / This dissertation examines the formation of Christian identity in Europe through the ritual performances of baptism. Baptism was an essential act of social and religious initiation experienced by the majority of people in Europe, yet historians have struggled to understand its administration for ordinary lay participants as Europe transitioned from paganism to Christianity. Rather than a uniform indicator of Christian identity as described in clerical texts and current scholarship, baptism changed dramatically between the sixth and twelfth centuries. I show how what began as a flexible array of diverse religious practices located in watery landscapes, Roman-style baptisteries, portable spoons, lead tubs, and wooden buckets, evolved into a ritual standardized in the stone baptismal font, a form which persists to this day. I deploy an interdisciplinary methodology that engages robustly with church archaeology and art history to demonstrate how baptism created localized religious identities for new converts through its use of diverse ritual places and things. This study challenges our definition of a united medieval Christendom by radically reinterpreting the long-term practice of baptism as a slow process of Christianization in Europe from below. / Thesis (PhD) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: History.