[summary of the book] All human societies know religions: the cults of the peoples of hunters (chamanism, totemism, animism), the polytheistic Antiquity cults found in Egypt, Greece and Rome, but also in the Incas, Azteques, Mayas, the spiritualities of the East (hindouism, taoism, Buddhism, etc.), major monotheistic religions (Judaism, Christianity, Islam). Where does the need to believe come from? How can we explain the ubiquitous presence of religions in human history? And what is a religion? In this book, the religious phenomenon is addressed on the basis of three complementary insights: • The first part wonders about the “why” of religions. The humanities provide multiple answers to this question. Sociologists and anthropologists invoke social causes. Psychologists and psysonists focus rather on the ‘need to believe’. • The second part explores the forms of religious phenomenon through history. From the religions of pre-history to modern churches and ‘great religions’, history has constantly recreated new configurations that adapt, become more stable, decline and return in turn. • The last part wonders about the links between religion and modernity. While the 19th century prophetised the ‘death of God’, which wanted religions to disappear, this seems to be resurgence today: revival of Islam, rise of evangelical protestantism re-emerging in China, etc.