One of the hallmarks of social science is the interaction of theory and methods/data, the former guiding the latter and the latter refining the former, in a cyclical relationship. The goal of theory is to provide explanations for and even predict a range of human behaviors. One potential cause of theoretical stagnation is an over focus on a singular, usually easily accessible group. Given the persistence of plain Anabaptists like the Amish as a highly distinct subgroup in American society, their utility for refining sociological theories is persuasive, but has rarely been employed to this end because of their social inaccessibility, shyness towards social science research, and the popular interpretive frames placed on them that distract would-be investigators. Even with Amish-focused scholarship, the emphasis has been largely on describing the population or applying theory to understand the Amish case, but not returning findings back to theory in critique and revision. This dissertation introduces and contextualizes the plain Anabaptists, then describes the Beachy Amish-Mennonites, a group within the Amish religious tension, but dealing markedly with tensions between separatism and assimilation. Following this introduction are three independent studies that demonstrate the use of plain Anabaptists to refine theory. The first study focuses on migration motivations. Rational choice theory is the dominant perspective in understanding migration causation: actors migrate to achieve personal ends at as low a cost as possible. Some migration may be motivated by values, such as religiously based migration. This study proposes a theory of religiously motivated migration. Inasmuch as values are derived from groups, religions with strong membership cohesion must maintain this cohesion in the face of emigration so members continue acting on value-based demands. Religious cohesion is maintained through community-level migration and affiliation-level networks, which both provide members with unbroken religious systemic integration after emigration. Three religious reasons for migration are identified: sacred command, context conducive for religious practice, and awareness of potential membership losses from religious competition. This theory is demonstrated through the case of domestic and international Amish-Mennonite migration. The second study focuses on subgroup tensions over assimilation. While numerous ethnic groups have assimilated into the American mainstream, the Amish church has embraced cultural and structural separatism on religious grounds. Elements of their cultural system are not just demarcations of social identity but direct members’ social ties, values, and interests inward, permitting the perpetuation of group socialization. However, some members may perceive a level of assimilation desirable and so pursue structural power and mobilization of external cultural resources. Because structural and cultural assimilation reinforce one another, when one weakens, the other may follow and further weaken the first. Separatist-assimilationist conflicts have dotted Amish history, most notably when progressively-oriented Amish-Mennonites have withdrawn from the Old Order Amish. While two past Amish-Mennonite movements assimilated over several generations, the most recent movement, the Beachy Amish-Mennonites, still retain a partially separate identity, though with some difficulty. Inasmuch as separatism must be maintained across generations, the orientation of Beachy young adults is of particular interest. This study investigates the social structure of a Beachy young adult network to determine what kinds of people occupy positions of power and its implications for assimilation of a third Amish-Mennonite movement. The results indicate that those who attempt to alter the content of, rather than replace and negate, separatist practices occupy positions of power, suggesting a third actor type in the separatist-assimilation conflict: revisionists. The third study focuses on mainstream Americans seeking to join the plain Anabaptists. For all the liberties granted Westerners, a small but regular stream of people seek to join seemingly austere plain Anabaptist sects (Amish, Mennonites, etc.). What are these “outsiders” seeking? I developed a survey to explore this question and posted it on a prominent Anabaptist website, offering outside seekers information about nearby churches in exchange for their time. Usable responses numbered 1,074 over two years. Evangelicals, Baptists, females, people in the Midwest and South, and the young were overrepresented. Strongest attractions include devout Christianity, strong community, and modesty. A factor analysis of 17 sources of information suggests groupings by mediated and direct information sources. A factor analysis of 21 attractions suggests six types of seeker interest, characterized by emphases on family, femininity, religious conviction, primitivism, social support, and returning to the group. Relationships between the six attraction factors and information sources, age and gender, U.S. region, and religious background and explored through regression analyses