Justice is both the aim and the intrinsic criterion of all politics. Politics is more than a mere mechanism for defining the rules of public life: its origin and its goal are found in justice, which by its very nature has to do with ethics. The State must inevitably face the question of how justice can be achieved here and now. But this presupposes an even more radical question: what is justice? The problem is one of practical reason; but if reason is to be exercised properly, it must undergo constant purification, since it can never be completely free of the danger of a certain ethical blindness caused by the dazzling effect of power and special interests.
Here politics and faith meet. Faith by its specific nature is an encounter with the living God—an encounter opening up new horizons extending beyond the sphere of reason. But it is also a purifying force for reason itself. From God's standpoint, faith liberates reason from its blind spots and therefore helps it to be ever more fully itself. Faith enables reason to do its work more effectively and to see its proper object more clearly. This is where Catholic social doctrine has its place: it has no intention of giving the Church power over the State. Even less is it an attempt to impose on those who do not share the faith ways of thinking and modes of conduct proper to faith. Its aim is simply to help purify reason and to contribute, here and now, to the acknowledgment and attainment of what is just.
The Church's social teaching argues on the basis of reason and natural law, namely, on the basis of what is in accord with the nature of every human being. It recognizes that it is not the Church's responsibility to make this teaching prevail in political life. Rather, the Church wishes to help form consciences in political life and to stimulate greater insight into the authentic requirements of justice as well as greater readiness to act accordingly, even when this might involve conflict with situations of personal interest. Building a just social and civil order, wherein each person receives what is his or her due, is an essential task which every generation must take up anew. As a political task, this cannot be the Church's immediate responsibility. Yet, since it is also a most important human responsibility, the Church is duty-bound to offer, through the purification of reason and through ethical formation, her own specific contribution towards understanding the requirements of justice and achieving them politically.
The Church cannot and must not take upon herself the political battle to bring about the most just society possible. She cannot and must not replace the State. Yet at the same time she cannot and must not remain on the sidelines in the fight for justice. She has to play her part through rational argument and she has to reawaken the spiritual energy without which justice, which always demands sacrifice, cannot prevail and prosper. A just society must be the achievement of politics, not of the Church. Yet the promotion of justice through efforts to bring about openness of mind and will to the demands of the common good is something which concerns the Church deeply.
--Benedict XVI, Deus Caritas Est, God is Love, Encyclical, 2005.