All societies and ideologies need a purpose, a comprehensive metaphor that explains the role of man in the universe. For the left, the calls for social justice have to be attached to a larger understanding of who we are and why we are here for them to have a mission that is about more than just band aids and sociological surveys. Social justice provides a social argument, and for those on the left who still believe in a creator deity, even a theological justification. But as the left largely trends secular, it needs a secular theology that lays out its mission. Environmentalism provides that theology.
