Charles Gunnoe's welcome biography illuminates a contentious intellectual whose sizable imprint upon sixteenth- and seventeenth-century learned culture has not received commensurate attention in recent scholarship. Although Erastus is introduced as a “Renaissance physician” in the book's subtitle, it is his contributions to theology and natural philosophy that receive the most attention. Based primarily upon correspondence and printed works, this study focuses upon Erastus's years at the University of Heidelberg, where he served as medical professor and rector between 1556 and 1580. In the early 1560s, Erastus also belonged to a circle of advisers who encouraged Frederick III, ruler of the Palatinate, to adopt Reformed Protestantism. As a theologian, Erastus fervently opposed independent church discipline courts, which supporters of Calvinism introduced to the Palatinate in 1570. While he did not prevail in the dispute, Erastus's ideas influenced later arguments for state control over religion that were advanced by Hugo Grotius, John Selden, and Thomas Hobbes.

In the first of three lengthy sections, Gunnoe charts the arrival of Reformed Protestantism and the intellectual alliance that culminated in the Heidelberg Catechism, a founding charter of German Reformed Protestantism that was first published in 1563. Gunnoe suggests that Erastus played an influential role in the work's composition, alongside Caspar Olevianus and Zacharias Ursinus. In particular, the doctrine of the Lord's Supper in the Catechism closely parallels a tract that Erastus published in 1562. But Erastus's influence did not last; as described in the work's second section, tensions among different reforming traditions erupted when supporters of Calvin advocated for a consistory that would have powers of excommunication. Erastus countered with the argument that there was no Biblical justification for the practice. This claim, and his association with more radical theologians, prompted a heresy inquest in 1574 and, apparently, Erastus's temporary suspension from communion around the same time.

The book's third section explores Erastus's shifting interests as his influence in theological matters waned. In 1572, he published De medicina nova, a four-volume refutation of the medical philosophy of Paracelsus. Gunnoe concludes that this monument of medical polemic probably did less to discredit Paracelsian ideas than to unwittingly introduce them to a new generation of curious readers. Erastus also ventured into the debate over witchcraft, arguing in support of capital punishment for witches against the pleas for clemency advanced by Johann Weyer in De praestigiis daemonum. While both men were sceptical that witches actually accomplished marvellous things, Erastus stressed the notion of a demonic pact, which he interpreted as a satanic inversion of the covenant between God and humans. Hence, witches were culpable more for their beliefs than their actions; as Gunnoe points out, this was a corollary of the Reformed emphasis on faith and the rejection of miraculous forces in either religion or magic.

The biography's narration of more or less distinct episodes lends the impression that Erastus shaped his views in response to controversies rather than as dimensions of an overarching natural philosophy. However, Gunnoe discerns a common theme with his concluding suggestion that Erastus's polemical writings “fought to establish the independence of the book of nature from the book of revelation” (415). Just as Erastus rejected the special claim to authority asserted by Calvinist pastors, his suspicion of occult or revealed knowledge motivated his attack on Paracelsus. Although this rather bulky volume is best suited to specialists, Gunnoe has interspersed broader contextual passages that will orient more casual readers. Author and publisher should be commended for including lengthy Latin quotations in the footnotes, a brief selection of translated letters, and a correspondence register that lists over 500 letters sent by Erastus to dozens of associates. Hopefully, diagrams of Erastus's correspondence networks and transcriptions of more letters, understandably omitted from the printed volume, will be posted online to complement the treatises in the Post-Reformation Digital Library and other internet resources.