Pederson's monograph makes a significant contribution to the vexed issue of delineating ‘Puritanism’. It attempts to navigate between the Scylla of nominalism that predicates only individual Puritans and moves from anatomising to atomising, and the Charybdis of monolithic Puritanism, purged of any dissent. Pederson draws on Wittgenstein's theory of familienähnlichkeit to enable both synchronic ‘family resemblance’ (unitas) whilst preserving individual diversitas. Influenced by Richard Muller, Pederson's approach fits within what Peter Lake termed the intrinsic school that privileges reformed confessional identity. However, Pederson also considers the extrinsic context of a movement that was but ‘one half of a stressful relationship’ (Collinson). As a heuristic tool, Unity in diversity offers a versatile approach for both intensive studies of individual Puritans and the expansive narratives that comprise Puritanism. After rehearsing various historiographical challenges, Pederson identifies four theological ‘strains’ within Puritanism (Precisianist, Mystical, Antinomian, Neonomian). Three Puritans are then considered as federal heads of the first three ‘strains’ – John Downame, Francis Rous and Tobias Crisp. In selecting these, Pederson road-tests his model by considering Unity in diversity for a practical predestinarian, a mystical politician and an anti-preparationist Antinomian. In doing so, he also sheds light on three less researched Puritans. Each is situated in his historical context before their writings are subjected to theological analysis according to certain loci: doctrine of God and humanity; predestination and assurance; covenant of works and grace; justification and sanctification; law and Gospel; Christian life and piety. Final chapters offer a nuanced conclusion: Puritanism is diverse and any comprehensive definition will require plasticity. However, all three Puritans remain broadly within the bounds of confessional Reformed Orthodoxy and committed to the Puritan vision of praxis pietatis. According to this comparative study, Puritanism retains sufficient unitas to avoid being hung drawn and quartered into disparate –isms. It is a measure of Pederson's critical eye that he also highlights shortcomings: ‘more work will need to be done on how unitas and diversitas relate to those Puritans who were neither strictly “Reformed” nor “orthodox”’ (p. 316). Tobias Crisp was considered ‘radical’ within the Puritan societas but when assessed by a model that privileges doctrinal criteria, he emerges surprisingly ‘orthodox’. This triangulated study side-steps Puritans from the fourth ‘strain’ (Neonomianism) who modify confessional soteriology (Richard Baxter), let alone ‘Puritans’ who tamper with Trinitarian creedal orthodoxy (John Milton). Pederson considers the challenge posed by ‘radical’ or ‘hybrid’ Puritans (p. 302) but avoids it by restricting his caseload to early Stuart Puritans (despite the dates in the title: ‘1603–1689’). This leaves crucial questions unresolved: What is the relationship between Puritanism and Reformed Orthodoxy? Should a divine who deviates from Reformed confessions still be considered ‘Puritan’? During the English revolution and republic, variance increases to doctrinal deviance as new strains emerge among the godly (for example the Puritan Arminian, John Goodwin). A comprehensive approach will need to engage with the full extent of synchronic and diachronic diversitas before passing sentence on the unitas of Puritanism.