Did changes in religious practice, content and discourse amount to a ‘feminisation’ of religion in nineteenth-century Europe? Tine Van Osselaer's insightful monograph addresses this question by examining Belgian Catholicism from the late eighteenth century until the Second World War. Rather than seeking to confirm the ‘feminisation thesis’, the author also investigates efforts to appeal to male audiences, driven by concerns about ‘men's negligence of Christian practices and lack of church loyalty’ (p. 68). In doing so, she sheds light on gender roles within the church, the household and lay movements. Van Osselaer focuses on a country where a variety of Catholic movements and tendencies – from Ultramontanism to Social Catholicism – displayed considerable vigour. Although the Catholic Party dominated Belgian politics from 1884 until the Great War, liberals and socialists actively challenged church power. This was a period of major conflict between secular and ecclesiastical forces and, within this context, Belgian Catholics developed a rich associational life. It therefore seems apt that Van Osselaer dedicates considerable space to Catholic lay organisations. Some of these bodies were linked to the Cult of the Sacred Heart. As early as 1868 the Belgian bishops consecrated their country to the Sacred Heart of Christ – more than three decades before Leo xiii’s encyclical Annum Sacrum did the same for humanity as a whole. The Sacred Heart was celebrated publicly through the construction of dedicated churches but also privately via ‘enthronement’ ceremonies in Belgian households. With its emphasis on emotional expression, Sacred Heart devotion was often deemed ‘feminine’; yet, as Van Ossalaer points out, it could also be cast as ‘martial’ (p. 129). This ambiguity manifested itself in the associational field. Initially, the Apostleship of Prayer played a leading role in promoting the devotion; whilst not gender-exclusive, it was largely led by women. Soon enough, however, broader anxieties regarding male religiosity triggered all-male ventures which, by the early twentieth century, were known as the ‘Leagues of the Sacred Heart’. In 1929 Pius xi praised the ‘masculine character’ (p. 138) of the Leagues. At the same time, he championed a hierarchical model of lay activism as exemplified by Catholic Action. With its ‘emphasis on authority, the idealisation of Christ as a leader and mass activities’ (p. 202), Catholic Action resonated with the politics of the period. As Van Ossalaer notes, the Leagues’ response to this development ‘cannot really be called enthusiastic’ (p. 180). None the less, there was cooperation on specific issues, for instance in the campaign for a ‘return to Sunday Mass’. Furthermore, Catholic Action was hardly a homogenous phenomenon: while a discourse of heroic masculinity prevailed in its male sections, Van Osselaer identifies different features in women's Catholic Action groups. As a whole, The pious sex draws a nuanced and multi-layered picture that helps to question terms such as ‘feminisation’ and ‘masculinisation’. The density of the prose requires some concentration – as does the author's tendency to deploy examples from different time periods in rapid succession. Yet, the book readily repays the reader's investment. It is a well-researched work; its considerable substance should speak to historians of both gender and religion.