In the opening chapter of The Protestant Ethic and the Spirit of Capitalism, titled "religious affiliation and social stratification" Max Weber notes how statistics show that in mixed religion societies protestant tend to rank higher in socio-economic stances. This for Weber links Protestantism with capitalism and he is seeking for an answer for the link between them. In other words, Weber argues that religious beliefs have to do with economic practices and socio-economic position (for additional elaboration see Max Weber's theory of stratification)
Throughout "religious affiliation and social stratification" Weber shows how differences between Catholicism and Protestantism can account for different professional and economical attitudes produced by different environments which are, as a result, more or less adapted to the capitalist system. Weber tries to account for these differences as the basis for his whole theory in of The Protestant Ethic and the Spirit of Capitalism.
One explanation offered for differences between Catholicism and Protestantism is that Catholic people are less concerned with material gains and are more focused on gains in the afterlife. But Weber thinks this does not fully account for the differences. He holds that the fact Protestantism works better with capitalism is due to an intimate connection between the two, making them two aspects of the same thing (hint: rationalism).
From this point in "religious affiliation and social stratification" Weber attempts to show how different features of Protestantism are adapted to, in fact yielded from, capitalism. His main argument which will stand at the core occupation of The Protestant Ethic and the Spirit of Capitalism is that the characteristics of a certain faith can transform into conditions of economic (and not just economic) personal and social behavior. An additional aspect of this theory is how it can link personal belief and religious belongingness to social statues and stratification.