From time immemorial, missionaries, especially Protestant ones, have endeavoured to count their converts, not least to reassure their home boards and donors that their financial investment has yielded a spiritual return. Rather more recent is the concern of scholars of what is now known as ‘World Christianity’ to map and quantify the global distribution of the Christian religion. Since 1982 their most frequently cited sources for such endeavours have been the three successive editions of the World Christian encyclopedia (1982, 2001, 2020) and now also the closely associated on-line World Christian Database hosted by the Center for the Study of Global Christianity at Gordon-Conwell Theological Seminary in the United States. The architect of the first edition of the Encyclopedia was David B. Barrett, an Anglican missionary working in Kenya with the Church Missionary (now Mission) Society (CMS). Zurlo's book originated as a doctoral dissertation at Boston University School of Theology. It tells the story of the genesis of the Encyclopedia and Barrett's often fraught relationship with his chief collaborators, the Belgian Catholic priest and sociologist François Houtart, and the American Evangelical missiologist Edward Dayton, who worked for the Missions Advanced Research and Communications Center (MARC) established by World Vision.

David Barrett was a Cambridge-educated aeronautical engineer who in 1952 began training for the Anglican ministry at Ridley Hall. After two years of ministry in Bradford, he offered for missionary service with the CMS and was sent to Nyanza province in western Kenya. Becoming aware of his lack of academic training in the study of African cultures and religions, in 1962 he took leave from his missionary service to pursue STM and then PhD studies at Union Theological Seminary and Columbia University in New York. On his return to Kenya, the (Anglican) Church of the Province of Kenya appointed him as its Research Secretary, and one notable fruit of this period was Barrett's first book, Schism and renewal in Africa (Oxford 1968), a continent-wide survey of the emergence of African Independent [or Instituted] Churches. Although not well received by the pioneering scholars who studied African Independent Churches in depth in their particular regional contexts – Bengt Sundkler, Harold Turner and Marthinus L. Daneel – the book established Barrett's reputation as a social scientific analyst of Christian movements on a macro scale.

Barrett's time in New York had exposed him to the then dominant empirical tradition of American sociology, with its heavy emphasis on surveys and quantification. Zurlo traces the origins of his vision for the quantification of World Christianity to the impact of American empirical sociology of religion on his thinking, and his intellectual indebtedness to this tradition is very clear. Nevertheless, the World Christian encyclopedia has a longer lineage. Its most prominent forerunners were three early twentieth-century works tabulating Protestant missionary achievement in both statistical and cartographical terms: James S. Dennis's Centennial survey of foreign missions (1902); the Statistical atlas of Christian missions, compiled by John R. Mott's Commission at the World Missionary Conference in 1910; and the World missionary atlas (1925), edited by Harlan P. Beach and Charles H. Fahs. It is a pity that Zurlo makes only brief allusions on pp. 32, 42, 59 and 103 to these notable forebears of Barrett.

The immediate precursor to Barrett's encyclopaedia was the series of five editions of the World Christian handbook produced by the unhappily titled World Dominion Press from 1949 to 1968. Indeed, the first World Christian encyclopedia in 1972 was originally planned as a sixth edition of the World Christian handbook, but on a broader ecumenical basis. On the initiative of Lesslie Newbigin, the project was conceived as an ecumenical partnership between Barrett and Houtart – a Marxist sympathiser and early enthusiast for theologies of liberation. The book was envisaged as a successor both to the Handbook and to the Catholic Bilan du monde, whose two previous editions in 1958 and 1964 had surpassed the Handbook in geographical scope. It was Barrett who continually expanded the brief for the project in the hope that it might become a Christian version of the Encyclopaedia Britannica. The result was that costs spiralled out of control, deadlines were repeatedly missed and relationships between Barrett, the publisher (Macmillan), Houtart and Dayton – who processed the data using the then infant technology of computerisation – all became frayed. Barrett also aroused the ire of his ecclesiastical superiors in the Research Unit of the Church of the Province, principally Archbishop Festo Olang’, as his research activities appeared to bring little tangible benefit to the Church of the Province, whose interests the unit had been established to serve. He ploughed on regardless, leaving his critics and collaborators floundering in his wake. One of the merits of Zurlo's book is that she reveals how far the World Christian encyclopedia was, necessarily, a corporate endeavour, even if Barrett took the credit: ‘he essentially cut out his co-editors and made himself the sole editor’ (p. 99).

The statistics presented in the Encyclopedia provoke multiple questions. Barrett adopted self-definition as the primary basis of his figures of religious affiliation, and there was no satisfactory alternative to that. However, by inflating figures for meaningful Christian affiliation in Europe, that basis had the effect, contrary to Barrett's intention, of softening the contrast with the burgeoning church growth of Africa, for example. Comparatively few Africans style themselves as Christians without actually attending church. Moreover, perhaps in response to this problem, he introduced into his statistics a problematic distinction between ‘professing Christians’, ‘affiliated Christians’ and (most untestable of all) ‘practising Christians’. Barrett had a touching but misplaced faith in the accuracy of denominational membership figures produced for public consumption (p. 79). He treated definitions of church membership as if they all referred to the same thing across the denominational spectrum, which is clearly not the case. The temptation of those schooled in social scientific methodology is to present as ‘fact’ what are in fact estimates, grounded more or less securely in hard evidence, and employing categories whose meaning may vary over place and time. That reality qualifies the usefulness of the World Christian encyclopedia and the continuing on-line database that it has generated. There are a few errors. Ridley Hall was never the divinity school of the University of Cambridge (p. 10), and the notable Leakey family in Kenya is misspelt as ‘Leaky’ (p. 19). Nevertheless, Gina Zurlo has performed a valuable service for scholars of world Christianity in her careful uncovering of the complex and sometimes morally ambiguous narrative that lies behind the production of this remarkable work of reference.