This chapter approaches the English Revolution as a theological crisis, a struggle over the identity of British Protestantism. During the mid-1640s, the Westminster Assembly laboured to reform the church and establish confessional orthodoxy, but despite producing a series of major documents, its presbyterian majority faced serious challenges from Erastians, Independents, and the growth of ‘sects and heresies’. While clergy like Richard Baxter, John Owen, Jeremy Taylor, and Richard Allestree produced works that soon acquired classic status, the Revolution also witnessed a spectacular proliferation of lay theology, ranging from high Reformed orthodoxy to the heterodox doctrinal systems of John Milton, Sir Henry Vane, and Thomas Hobbes. England became a marketplace of competing religious ideas, and religion exercised a powerful influence over political and scientific discourse. Although England’s ‘puritan Revolution’ ended in failure, the religious thought of this period was to prove seminal for later Calvinists, Anglicans, and Quakers.