Historians of early modern England have long recognised the significant ways in which eschatological speculation impinged upon the period. While a previous generation including Christopher Hill, Bernard Capp and William Lamont tended to interpret apocalyptic and millennial rhetoric as a proxy for wider socio-economic and political concerns, more recent scholars such as Crawford Gribben, Andrew Crome and Martyn Cowan have shown far more interest in the theological categories and thought patterns employed by the seventeenth-century actors themselves. John Duff's ‘A knot worth unloosing’ makes a fresh contribution to this latter school by turning his attention to early modern understandings of the biblical phrase ‘the new heavens and new earth’. Despite occurring just four times in the Bible, the idea represented by the phrase is more pervasive, and, as Duff notes, it points to an important eschatological theme that has often been overshadowed by the ‘scholarly fixation upon the rejuvenation of millennialism’ during the seventeenth century (p. 228). The book demonstrates that early modern English interpretations of the ‘new heavens and new earth’ fell broadly into two interpretive streams, one which understood the phrase ‘as a metaphor for changes in the civil and religious fortunes of the church in the present age’, and another that ‘treated the phrase more literally’ as referring to ‘a renovation of the material world at the consummation of all things’ (p. 221). Duff does a fine job meticulously tracing this theme through a range of published primary source materials, including commentaries, treatises, catechisms and sermons. In addition to treating figures like Joseph Mede (1586–1639) whose expositions of biblical prophecy are relatively well-known, Duff also interrogates more obscure figures such as the Devon minister John Seager and the theologically eclectic yet influential Baptist Thomas Collier (d. 1691). By assembling this diverse range of interlocutors, Duff documents that ‘the various interpretations of the new heavens and earth that existed in seventeenth-century England did not necessarily follow ecclesiastical or political allegiances’ (p. 32). Often, it should be noted, these close readings are very close indeed. For example, whilst examining a 1647 catechism composed by the otherwise obscure John Alcock, Duff presents a table in which he catalogues every scriptural passage referenced in the catechism, all sorted according to the paragraph in which they originally appeared (pp. 180–1). Of course, such careful historical spadework will be appreciated by many specialists, but more general readers should be duly warned that this volume presents a very detailed examination of key primary sources. In terms of Duff's historiographical interaction, one might have expected a bibliography that better reflected the current state of early modern eschatological research; despite a 2019 publication date, the book only cites five works published within the last fifteen years, and the newest work cited was published in 2011. Yet, despite this quibble, ‘A knot worth unloosing’ offers readers a lucid, clearly written and straightforwardly organised exposition of a key yet understudied early modern eschatological concept.