This dissertation argues that Thomas Grantham (1633/4-1692) was instrumental in organizing and legitimizing the General Baptists in Lincolnshire and Norfolk in the second half of the seventeenth century. The first chapter introduces the study and provides a historiographical survey of the variety of ways in which Grantham has been studied. The second chapter provides a biographical and literary sketch of Grantham’s life and published documents. Unlike most surveys of Grantham’s writings which focus on select publications, the literary sketch in the second chapter provides a brief and basic introduction to all of Grantham’s writings. The third chapter of this dissertation builds on the conclusions of J. F. V. Nicholson by examining Grantham’s role in consolidating the office of Messenger and establishing it as a distinctive third ministerial office among the General Baptists in the seventeenth century. Grantham helped to solidify the office of Messenger between 1660 and 1700 by publishing defenses of the Messenger’s office, preaching, baptizing, planting churches, and ordaining ministers in Lincolnshire and later in Norfolk. Chapter Four examines Grantham’s apologetic efforts to defend the baptized believers against Anglicans, Catholics, Quakers, and Presbyterians. He was called upon to contend with other Christian groups by means of public debates and epistolary correspondence. The fifth chapter addresses Grantham’s interactions with the government and his thoughts on civil matters. He represented the baptized believers of Lincolnshire before Charles II and called all Christians to remain loyal, peaceable subjects. Grantham affirmed the acceptance of government-issued licenses to preach and congregate for religious purposes, and he unequivocally instructed his readers to avoid revolution or sedition. Grantham argued that the baptized believers supported the welfare of England by paying taxes; he even believed Christians could hold positions in civil government and serve in the military.