Three main issues were of central concern to Max Weber: the role of ideas in history and social reality, the nature of power and power structures, and the methodology of the social sciences (see his notions on ethnomethodolgy).
"The Protestant Ethic and the Spirit of Capitalism" has to do with Max Weber's first concern – the manner by which ideas and perceptions function in history. Central to these preoccupation is the notion of rationality and rationalization in modernity.
The Protestant Ethic and the Spirit of Capitalism deals with the rise of capitalism in the wake of the protestant revolution. Weber's agenda is to show how a religious or ideological process can be associated with a political and/or economic one. But what is more important about The Protestant Ethic and the Spirit of Capitalism is the manner in which Weber sees capitalism and Protestantism as part of a large process of "rationalization".
Unlike Karl Marx, Weber views capitalism not as the central material feature of modern western societies but as one aspect of a larger process of rationalization which has come to dominate modern thought and society. Weber's discussion of these trends can be found in his work on both ideas and power, capitalism, Bureaucracy and Protestantism are all mutual aspects of modern rationalization.
Weber never really defines "rationalization,". One possible formulation of what Weber intended in "rationalization" is the peruse of a specific goal in a self-conscious and systematically organized manner. More simply, 'rational' according to Weber is the practice of appropriating means to an end.
Weber did not use “rationalization” and “rational” as necessarily terms of praise. He was quite ambivalent and critical about rationally organized action and a totally rationalized society as the ultimate goal of modernity. Consequently, his account of the uniquely rational features of western societies at the beginning of The Protestant Ethic and the Spirit of Capitalism does not imply that western societies are better than non-western ones, only different in their orientation and practice.
Rationality and rationalization are, according to Weber, inherit features of modernity which strives for a governed, calculated utilitarian and forward-moving modes of personal and social life.
Suggested reading by and about Max Weber and his theories: