The Compendium of the Social Doctrine of the Church names solidarity as a core principle of Catholic Social Teaching:
Solidarity highlights in a particular way the intrinsic social nature of the human person, the equality of all in dignity and rights and the common path of individuals and peoples towards an ever more committed unity. … The acceleration of interdependence between persons and peoples needs to be accompanied by equally intense efforts on the ethical-social plane, in order to avoid the dangerous consequences of perpetrating injustice on a global scale.
Many associate the term “solidarity” with the writing of Saint John Paul II, who developed the concept extensively and applied this key principle of social life to events in the final decades of the Cold War and in the years just after the fall of the Berlin Wall. The term, however, had been employed decades earlier by the participants in the Second Vatican Council (among them, a young Polish bishop who would become Pope John Paul II in 1978). In the social teaching document Gaudium et Spes, Vatican II described solidarity as the key to understanding several important contemporary realities: universal interdependency (in no. 4 of that document), international relations (in no. 90), and the communal character of the human person taught by Jesus Christ, the community of believers which he establishes, as well as the ultimate solidarity to be “brought to perfection” at the end of time (no. 32). In writing the social encyclical Pacem in Terris during the Council, Saint John XXIII called for an “active solidarity” which “cannot be divorced from the common good of the entire human family” (no. 98).
In developing the term “duty of solidarity,” John Paul II underscores the urgency of connecting action for justice to faith. For him, solidarity is the structural response demanded by gospel love. Solidarity, as a social principle, involves fundamental economic and social changes. In addition, in a striking assertion, the Polish pope says, “Solidarity is undoubtedly a Christian virtue.”
Solidarity therefore must play its part in the realization of this divine plan, both on the level of individuals and on the level of national and international society. The “evil mechanisms” and “structures of sin” of which we have spoken can be overcome only through the exercise of the human and Christian solidarity to which the church calls us and which she tirelessly promotes. Only in this way can such positive energies be fully released for the benefit of development and peace.
What is this solidarity that the pope speaks of?
John Paul’s answer connects us to the basic theme of the preferential love of the poor, a theme we hear anew from Pope Francis, or, as John Paul puts it, “God’s beloved poor”:
It is above all a question of interdependence, sensed as a system determining relationships in the contemporary world in its economic, cultural, political and religious elements, and accepted as a moral category. When interdependence becomes recognized in this way, the correlative response as a moral and social attitude, as a “virtue,” is solidarity. This then is not a feeling of vague compassion or shallow distress at the misfortunes of so many people, both near and far. On the contrary, it is a firm and persevering determination to commit oneself to the common good, that is to say, to the good of all and of each individual because we are all really responsible for all.
This solidarity takes concrete form, John Paul II says, in personal decisions, in “decisions of government” (no. 9), in economic decisions (no. 39), in public demonstrations by the poor themselves (no. 39), in sacrifice of all forms of economic, military, or political imperialism (no. 39), and in a variety of other concrete actions, both personal and structural. Solidarity, we are told by the Vatican, will require developing new forms of collaboration among the poor themselves, between the poor and the rich, among and between groups of workers, and between private and public institutions.
 Pontifical Council on Justice and Peace, Compendium of the Social Doctrine of the Church (2005), No. 192 (emphasis in original)..
 “The principle of social solidarity suggests that alleviating poverty will require fundamental changes in social and economic structures that perpetuate glaring inequalities and cut off millions of citizens from full participation in the economic and social life of the nation. The process of change should be one that draws together all citizens, whatever their economic status, into one community.” National Conference of Catholic Bishops, Economic Justice for All, 1986, no. 187.
 Pope John Paul II, Sollicitudo Rei Socialis, 1987, no. 40.
 Ibid., no. 38.
 Congregation for the Doctrine of the Faith, Instruction on Christian Freedom and Liberation, no. 57, March 22, 1986 (Washington, DC: U.S. Catholic Conference), no. 89, p. 53.