Assurance was a central issue for the eminent Scottish theologian-pastor Thomas Boston (1676–1732) long before it emerged as a focal point of the theological debate in the Marrow Controversy. Despite the sensation sparked by the republication of The Marrow of Modern Divinity and its enduring influence on Boston and Scottish church history, scholarship until now has not systematically examined the theology of the Marrow, let alone cross-analyzed it with Boston’s massive notes. While past studies have rightly stressed Boston’s federal theology and the fourfold state, they have not sufficiently underscored the centrality of union and communion with Christ in his theology, much less studied how this controlling theme impinges upon his doctrine of assurance and other theological structures. Scholars moreover have focused on assurance’s relation to faith, leaving other critical dimensions of assurance underexplored or even unexplored in the case of his sacramentology and its instrumentality for assurance. By situating Boston in his historical—especially his often overlooked intellectual and theological—milieu, this research presents the first full-length, multidimensional study of Boston’s theology of assurance in its Scottish context. This thesis argues that Boston’s doctrine of assurance is as eclectic as it is Christocentric, centered on union and communion with Christ, the architectonic principle of his theology. This study challenges the common, if uncritical, conception that Boston’s theology merely follows Calvin, the Scots Confession, the Marrow, the Westminster Standards, and Scottish federalism. Although Boston’s theology in broad strokes is Calvinistic and Westminsterian, exhibiting features of the Marrow, English Puritanism, and Scottish Presbyterianism, this study will demonstrate that Boston, nonetheless, unapologetically diverges from all these sources at crucial junctures. As such, he influences theologians contemporary with and after him. Boston, most strikingly, holds in tension assurance as intrinsic to faith—itself a gift from God’s sovereignty in election—while insisting on self-examination as a human responsibility. This salient mark of his doctrine of assurance originates from his assertion that Christ died for the elect alone but all—elect or not— have a warrant to receive Christ, who is, in Boston’s appropriation of the Marrow, the Father’s “deed of gift and grant to mankind lost.” As such, assurance is, theologically, a divine gift and, pastorally, a human endeavor. Certainty is thus both extra nos and intra nos. Given his eclectic, poised view on certainty, Boston may justifiably be seen as a theologian and pastor of assurance. This study presents original research of Boston’s theology of assurance in five chapters. Chapter 1 is a first-ever intellectual biography of Boston. It furnishes a fresh and rounded picture of Boston in his Scottish context and controversies, situating him in his historical—especially his often neglected intellectual and theological—milieu. Chapter 2 cross-studies the theology of the Marrow with Boston’s notes, tracing for the first time the eighteenth-century Scottish Marrow Controversy to its seventeenth-century English roots. It offers new insights into the points of continuity and discontinuity between Boston’s theology and the Marrow. The bulk of the thesis is a pioneering, systematic-theological analysis of the various dimensions of Boston’s notion of assurance. Chapter 3 deals with the trinitarian, covenantal, and Christological dimensions, probing Boston’s understanding of the covenants of redemption, works, and grace and their interaction with union and communion with Christ, leading to an alternative interpretation of the Marrow Controversy through the lens of union with Christ rather than federal theology. Chapter 4 explores the soteriological dimension entailing election, effectual calling, regeneration, conversion, faith, repentance, justification, adoption, sanctification, perseverance, and glorification. It studies how Boston employs marks and theological syllogism as means of assurance. It also positions assurance as never before among the forensic, relational, transformative, and eschatological elements of Boston’s ordo salutis. Chapter 5 considers the ecclesiastical and sacramental dimensions of Boston’s theology of assurance, hitherto unresearched, delineating how he views baptism and the Lord’s Supper as visible aids of assurance. Boston, this thesis reveals, has a potent and enduring power to speak on the perennial issue of assurance, rooted in union and communion with Christ, whom he considers as being the covenant itself