Thomas Goodwin has been described as 'the forgotten man of English theology' and, though known by some as a pioneer of congregationalism and a prominent member of the Westminster Assembly, the true significance and scope of his life's work has only recently been discovered. Historical reassessment has uncovered that the majority of Goodwin's treatises were intended to form a grand project defending Reformed soteriology in the 1650s against new threats as well as traditional opponents. Examining Goodwin's notion of union with Christ in relation to mystical indwelling, transformation, justification and participation, this study demonstrates the central role of union with Christ in Goodwin's soteriology. The application of salvation, he contended, must be founded on 'real' union with Christ (i.e., mystical union forged by Christ's indwelling) in order to advance a trinitarian, federal, high Reformed soteriology in which redemption from sin is set within a Reformed scheme of Christocentric deification. This in-depth analysis makes a fresh contribution to recent controversy over union with Christ in the post-Reformation period.