Why does rauschenbusch argue that "the salvation of individuals" is not sufficient to address social problems?

Rauschenbusch argues that "the salvation of individuals" is not sufficient to address social problems. This is because the salvation of individuals only focuses on saving people "one by one." This ignores the source of the social problems. Social problems receive this name because they are problems that are shared by all of society. They exist as a consequence of a systematic problem, and not an individual disadvantage. Therefore, if we want to address  these problems successfully, we need to think of people as groups, and not as separate individuals.

500 books can be produced by one man and a printing press within 5 months, while a copyist can only produce one in the same amount of time

for this case we have the following equation:

we apply distributive property to the terms within parentheses.

subtracting 2 on both sides of the equation: