The just ordering of society and the State is a central responsibility of politics. As Augustine once said, a State which is not governed according to justice would be just a bunch of thieves: "Remota itaque iustitia quid sunt regna nisi magna latrocinia?".[18] Fundamental to Christianity is the distinction between what belongs to Caesar and what belongs to God (cf. Mt 22:21), in other words, the distinction between Church and State, or, as the Second Vatican Council puts it, the autonomy of the temporal sphere.[19] The State may not impose religion, yet it must guarantee religious freedom and harmony between the followers of different religions. For her part, the Church, as the social expression of Christian faith, has a proper independence and is structured on the basis of her faith as a community which the State must recognize. The two spheres are distinct, yet always interrelated.

The Church can inform and influence politics by "purifying reason", but cannot impose itself on society.  

Benedict then goes on to criticize Marxism, which, in his view, ignores the suffering of the here and now while marching towards a utopian future:

The modern age, particularly from the nineteenth century on, has been dominated by various versions of a philosophy of progress whose most radical form is Marxism. Part of Marxist strategy is the theory of impoverishment: in a situation of unjust power, it is claimed, anyone who engages in charitable initiatives is actually serving that unjust system, making it appear at least to some extent tolerable. This in turn slows down a potential revolution and thus blocks the struggle for a better world. Seen in this way, charity is rejected and attacked as a means of preserving the status quo. What we have here, though, is really an inhuman philosophy. People of the present are sacrificed to the moloch of the future--a future whose effective realization is at best doubtful. One does not make the world more human by refusing to act humanely here and now. We contribute to a better world only by personally doing good now, with full commitment and wherever we have the opportunity, independently of partisan strategies and programmes.

Benedict's model for Christian engagement in society is Mother Teresa rather than Martin Luther King. He has always opposed Liberation Theology, and he makes no concessions to it here.  But there is much food for thought in this first encyclical, and evangelical Christians in America will be disturbed by much of what is here, including this:

Charity, furthermore, cannot be used as a means of engaging in what is nowadays considered proselytism. Love is free; it is not practised as a way of achieving other ends