Between the years 800 and 1200, Latin Christendom underwent a radical period of expansion as new lands and peoples were Christianized. This period included the conversion of Scandinavia to Christianity and the Baltic Crusades against Slavic peoples. The means of converting individuals and groups changed dramatically over the course of these few centuries. This thesis demonstrates that paying close attention to the various forms of institutional support that missionary activities received, as recorded in contemporary accounts from across the period, reveals previously unnoticed factors that made conversion efforts possible. The changing availability of institutional support however, depended directly on the shifting nature of political and ecclesiastical institutions within Latin Christendom, changes which would forever alter the realms beyond its boundaries. In particular, the development of canon law, crusade ideology, sacral kingship, and the increasing power of the state made possible military, financial, and spiritual support of missionaries in ways inconceivable before the year 1100. These developments help explain the transformation of the missionary bishop from wandering preacher to holy warrior, as well as sharpening the contrast between the peaceful conversion of Scandinavia around the year 1000 and the coercive conversion of the Baltic Slavs around the year 1200.