Weber’s focus in The Protestant Ethic and the Spirit of Capitalism is on the economic development of capitalism rather than on the Christian religion. However, inasmuch as he argues that modern Western capitalism was a consequence of the Protestant Reformation, particularly Calvinism, he discusses other Christian concepts and practices only when those themes had a relationship to the root or seed that resulted in the forest, or jungle, of modern capitalism. For him, the key figure is Calvin and the key concept is predestination, which logically flows from the definition of God as all powerful and all knowing, a description that encompasses the God of all of Western monotheism, including Judaism and Islam as well as Christianity. Saint Augustine and other early church fathers included predestination in their theological analyses, but it became the central pillar for Calvin, who did not shy away from its stark consequences, which logically led to the denial of human beings having free will.
If Calvin and Calvinism is the major theme in The Protestant Ethic and the Spirit of Capitalism, Weber also discusses, in part for purposes of comparison, the nature of medieval Catholicism, particularly the religious practices of the monastic orders who chose to retreat from the world in a quest for God rather than engage the world as, Weber argues, the Calvinists did. He also discusses the beliefs of Martin Luther and his salvation by faith, although unlike many historians of Luther, he finds Luther closer to Catholicism than Protestant Calvinism.
Finally, some economists and historians, notably R. H. Tawney, suggest that Weber got the connection between Calvinism and capitalism backward, and that it was the economic changes of the late Middle Ages and the Renaissance that required a new religious response to the traditional corporatism of Catholic civilization.