Assurance was a central issue for the eminent Scottish theologian-pastor Thomas Boston long before it emerged as a focal point of the theological debate in the Marrow Controversy. In The Marrow of Certainty, Chun Tse presents the first full-length study of Boston's theology of assurance in six dimensions: trinitarian, covenantal, Christological, soteriological, ecclesiastical, and sacramental. This work not only furnishes the first-ever intellectual biography of Boston in his Scottish context and controversies, but it also cross-studies the theology of the Marrow of Modern Divinity with Boston's notes. This research argues that Boston's doctrine of assurance centres on union and communion with Christ, the architectonic principle of his theology. The book challenges the common conception that Boston's theology merely follows Calvin, the Scots Confession, the Marrow, the Westminster Standards, and Scottish federalism. 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 the warrant to receive Christ. As such, assurance is, theologically, a divine gift and, pastorally, a human endeavour. Certainty is thus both extra nos and intra nos. Boston, this study reveals, has a potent and enduring power to speak on the perennial issue of assurance, rooted in the person of Christ, whom he considers as being the covenant itself.