Monotheism expanded its boundaries through the middle ages, as a common myth, its advantages against polytheism were not only its military and political agents but also its social and cultural gadgets. Up until now rates of these elements in religious transformation history kept secret. Thus, in the book comparative analysis of two examples had chosen to reveal its mysterious past. As the book tried to show, to understand religions and their transformative instincts, comparisons were inevitable. The book recounts the Vikings’ and Indians’ religious transformation by the hands of merchants, commandants, statesmen, clergies and missionaries. It was also comparing methods and efficiencies of Western Christendom and Muslim Orthodoxy in their spreading areas. Numerous chronicles, primary sources, firsthand testimonies and modern studies had contributed comparative analysis of Selahattin Ozkan, a scholar who spent his time to explore and examine many aspects of religious transformations in the middle ages.