Aim. The purpose of the paper is to determine criteria for assessing the religious affiliation of the population of various countries and regions, as well as reveal their advantages and disadvantages for human geography investigations. Methodology. Various sources containing statistical information about the religious affiliation are considered. The generalization is used to combine the sources into groups according to the generality of approaches, level of reliability and degree of objectivity. Use is also made of descriptive, historical and statistical methods. Results. Two main criteria for assessing the religious affiliation are identified: (i) ethnic and (ii) cultural and civilizational. In the first case, data sources rely on the condition of the identity of ethnic and religious affiliation, which is currently extremely rare. In accordance with the second criterion, which is more valid in modern conditions, the sources determine the individual’s religious affiliation based on answer to the corresponding question of the census or sociological survey. As a third, reserve criterion (in the absence of data based on the first two), an assessment of religious affiliation through the level of religiosity of the population is proposed. Research implications. The generalization of analytical materials on the religious affiliation of the population of countries and regions allows us to conduct comprehensive study on the geography of religion and to predict the development of the global religious landscape. An assessment of the number of adherents of various religions, conducted by the authors on the example of such a multi-religious and multi-ethnic country as Russia, makes it possible to develop recommendations for organizations involved in state-church regulation and the formation of policy frameworks in the field of national relations and migration processes.