Final answer:
Karl Marx emphasized the importance of religion in society, viewing it as a reflection and reinforcement of social stratification and economic conditions, despite his personal lack of religious belief.
Step-by-step explanation:
The individual who stressed the importance of religion as an institution for society, even though he was an atheist, is the German philosopher Karl Marx. Despite his personal disbelief in religious doctrines, Marx highlighted that religion plays a critical role in society, reflecting social stratification and maintaining inequality. Although Marx regarded religion as "the opium of the people," he recognized its integral role in societal structures, serving as an extension of economic conditions which perpetuated the status quo among different classes, especially the working class (proletariat).
Religion's significance, according to Marx, cannot be extricated from the economic suffering of the proletariat as it possesses a socio-economic dimension that cements the existing social order. This perspective posits that while religion may provide comfort, it also reinforces economic disparities by imparting a passive acceptance of conditions among the oppressed classes. Therefore, this understanding elucidates the complex interplay between religion and capitalist society, which Marx argued could not be dissected independently of one another.