Dictionary.com defines solidarity as (in definition 1):
union or fellowship arising from common responsibilities and interests, as between members of a group or between classes, peoples, etc.:
Solidarity, in its natural state, then, may be described as a sense of identification and common cause with others in their responsibilities and interests. Interests, in this case, should be construed in the broadest sense as entailing not just curiosity, but personal investment or need. Such solidarity is based, typically, on an existing relationship that arises from common membership in a group (such as a church parish, a guild — including a craft, technical, military or other professional organization — or a fraternal or sororal organization), a social or economic class or an ethnic, national, gender or religious commonality. "Union or fellowship with" implies sharing those "common responsibilities and interests."
Solidarity, as understood by the Catholic Church, focuses on our common humanity and our sharing of and interests in the natural resources of planet earth and in the richness of cultural heritage. Thus, it both elevates and broadens the natural definition through a recognition that we are all created by God to inhabit the same planet and to grow and develop together in the same stream of time.1
Jesus puts this in concrete terms in the parable of the good Samaritan.2 Jesus uses this parable to answer a question posed by a scholar of the Jewish Law. The question, "Who is my neighbor?" Can be rephrased in our terms as "With whom should I be in solidarity?" Jesus' answer is, in effect, "with anyone you find is in need."
Jesus' parable specifically incorporates a surprise element, namely that of a Samaritan, a member of a different national, religious and maybe even ethnic affiliation than the robber's victim, the "one in need." Note, too, that Jesus adds nothing to the parable that distinguishes anything we might call "identity" in today's common parlance, of the man who is a victim of robbers. Jesus' clear intent is to focus on the "man" solely as a human being in need.3 Jesus' parable, then, tells us that God looks beyond the accidental differences that tend to separate us and reminds us of our common existential identity.
We should note, too, that Jesus focuses on the directly personal, the man in need and the good Samaritan who chooses, our of kindness, to help him in whatever way he can. There is nothing in any of his parables that elevates the common interests of ethnic or religious groups. Indeed, Jesus' teaching elevates faith in him and the Gospel above all other points of identification. Consider, for example, Matthew 12:46-50, Matthew 15:21-28, two faith encounters in Luke 7:1-10 and 11-17. Even without considering faith, Jesus declares his personal sense of identification with everyone in need in Matthew 25:31-46, excusing no one who fails to appreciate this sense of solidarity with suffering humanity.
St. Teresa of Calcutta, better known as Mother Teresa, the "saint of the gutters", lived a life of service to the most forgotten and neglected of the earth. In her devotion to Jesus and in her prayer of adoration, she recognized this divine solidarity with all of humanity, particularly those who are in the greatest need. Seeing a stranger, abandoned by family and community, left to die in the very gutters of the city of Calcutta, she saw "Jesus in a distressing disguise." In every case, she was motivated, like the Good Samaritan, by a personal encounter, an encounter that moved her heart, an encounter that others (like the priest and the Levite in the parable of the good Samaritan) would have gone out of their way to avoid.
Indeed, Jesus declared that we should even love our enemies and pray for them. See Matthew 5:43-48 and Luke 6:27-28.
St. Paul recognized the super importance of our common humanity, especially in our identity as children of the one God. [Galatians 3:26-29]
For through faith you are all children of God in Christ Jesus.
For all of you who were baptized into Christ have clothed yourselves with Christ.
There is neither Jew nor Greek, there is neither slave nor free person,
there is not male and female;
for you are all one in Christ Jesus.
And if you belong to Christ, then you are Abraham's descendant, heirs according to the promise.
Given that God does not rank us according to our identity in the world, by what does he rank us? We find the answer in the Gospel for September 26, 2017. [Luke 18:19-21]
The mother of Jesus and his brothers came to him
but were unable to join him because of the crowd.
He was told, "Your mother and your brothers are standing outside
and they wish to see you."
He said to them in reply, "My mother and my brothers
are those who hear the word of God and act on it."
Are we faithful children of God? How faithful? What challenges will we overcome to remain faithful? Answers to questions like these matter to our eternal salvation. What associations we belong to, by comparison, do not matter.
Clearly, solidarity is seen here as a virtue to be cultivated, not a value to be imposed. The Church, taking this a step further, declares that government must function in support of both solidarity and subsidiarity. To clarify what this means, let us first refer to Part IV, b of the Compendium. In particular, in ¶ 351 and 352, we find…
 The action of the State and of other public authorities must be consistent with the principle of subsidiarity and create situations favourable to the free exercise of economic activity. It must also be inspired by the principle of solidarity and establish limits for the autonomy of the parties in order to defend those who are weaker. Solidarity without subsidiarity, in fact, can easily degenerate into a “Welfare State”, while subsidiarity without solidarity runs the risk of encouraging forms of self-centred localism. In order to respect both of these fundamental principles, the State's intervention in the economic environment must be neither invasive nor absent, but commensurate with society's real needs. “The State has a duty to sustain business activities by creating conditions which will ensure job opportunities, by stimulating those activities where they are lacking or by supporting them in moments of crisis. The State has the further right to intervene when particular monopolies create delays or obstacles to development. In addition to the tasks of harmonizing and guiding development, in exceptional circumstances the State can also exercise a substitute function”.
 The fundamental task of the State in economic matters is that of determining an appropriate juridical framework for regulating economic affairs, in order to safeguard “the prerequisites of a free economy, which presumes a certain equality between the parties, such that one party would not be so powerful as practically to reduce the other to subservience”. Economic activity, above all in a free market context, cannot be conducted in an institutional, juridical or political vacuum. “On the contrary, it presupposes sure guarantees of individual freedom and private property, as well as a stable currency and efficient public services”. To fulfil this task, the State must adopt suitable legislation but at the same time it must direct economic and social policies in such a way that it does not become abusively involved in the various market activities, the carrying out of which is and must remain free of authoritarian — or worse, totalitarian — superstructures and constraints.
It should be noted that the two virtues complement each other. Solidarity and subsidiarity are both virtues to be cultivated in a society, such as a family, a neighborhood, a community, a city, a state, a nation and the world. Solidarity unites people to achieve an objective, such as promoting the common good or the good of specific people in need. Subsidiarity recognizes and honors the inherent dignity of individuals, families, neighborhoods, etc. in their capacity to serve their common good and the special needs of their members, with greater levels of community (particularly those organized by a formal governing relationship) stepping in to assist lower levels only when the latter genuinely need that assistance, and, in doing so, acknowledge the legitimate role of the receiving individuals or communities to shape the nature of that assistance.
A society which practices the virtues of solidarity and subsidiarity gives room for its members (including constituent communities) to grow and develop in an integral and distinctly human way. This means that all aspects of each member are honored, particularly the spiritual as well as the physical. Human conscience is honored. The capacity (however limited or expansive) to contribute to the common good is honored, with the implication that there is an expectation of cooperation when common goods and needs are identified. When this perspective is fully appreciated, one reaches a balanced perspective on the value and purposes of a free market system, as well as the natural limitations that come into play in the exercise of free economic activity.
"Okay," you say, "but where does this idea of subsidiarity come from? Where does scripture mention it?" Scripture does not mention '"subsidiarity", as such, but Jesus' instruction to his disciples lays the groundwork.
But Jesus summoned them and said, “You know that the rulers of the Gentiles lord it over them, and the great ones make their authority over them felt. But it shall not be so among you. Rather, whoever wishes to be great among you shall be your servant; whoever wishes to be first among you shall be your slave. Just so, the Son of Man did not come to be served but to serve and to give his life as a ransom for many.”
Thus was born the ideal of servant leadership. Over the centuries since then, the Church has grown in its understanding of the application of this principle in the context of natural law. In 1891, Pope Leo XIII, writing in response to the growing unrest between labor and capital, reflected on the natural authority of the family in relation to the government in his encyclical Rerum Novarum…
 The contention, then, that the civil government should at its option intrude into and exercise intimate control over the family and the household is a great and pernicious error. True, if a family finds itself in exceeding distress, utterly deprived of the counsel of friends, and without any prospect of extricating itself, it is right that extreme necessity be met by public aid, since each family is a part of the commonwealth. In like manner, if within the precincts of the household there occur grave disturbance of mutual rights, public authority should intervene to force each party to yield to the other its proper due; for this is not to deprive citizens of their rights, but justly and properly to safeguard and strengthen them. But the rulers of the commonwealth must go no further; here, nature bids them stop. Paternal authority can be neither abolished nor absorbed by the State; for it has the same source as human life itself. "The child belongs to the father," and is, as it were, the continuation of the father's personality; and speaking strictly, the child takes its place in civil society, not of its own right, but in its quality as member of the family in which it is born. And for the very reason that "the child belongs to the father" it is, as St. Thomas Aquinas says, "before it attains the use of free will, under the power and the charge of its parents."(4) The socialists, therefore, in setting aside the parent and setting up a State supervision, act against natural justice, and destroy the structure of the home.
This recognition of the natural authority of the family within its own domain, together with the subsidiary role of support or correction by the state, is the start of the Church's development of the principle of subsidiarity from the principle of servant leadership. Pope Pius XI developed the idea further in his encyclical Quadragesimo Anno…
 As history abundantly proves, it is true that on account of changed conditions many things which were done by small associations in former times cannot be done now save by large associations. Still, that most weighty principle, which cannot be set aside or changed, remains fixed and unshaken in social philosophy: Just as it is gravely wrong to take from individuals what they can accomplish by their own initiative and industry and give it to the community, so also it is an injustice and at the same time a grave evil and disturbance of right order to assign to a greater and higher association what lesser and subordinate organizations can do. For every social activity ought of its very nature to furnish help to the members of the body social, and never destroy and absorb them.
 The supreme authority of the State ought, therefore, to let subordinate groups handle matters and concerns of lesser importance, which would otherwise dissipate its efforts greatly. Thereby the State will more freely, powerfully, and effectively do all those things that belong to it alone because it alone can do them: directing, watching, urging, restraining, as occasion requires and necessity demands. Therefore, those in power should be sure that the more perfectly a graduated order is kept among the various associations, in observance of the principle of subsidiary function, the stronger social authority and effectiveness will be the happier and more prosperous the condition of the State.
Further developments of the principle of subsidiarity can be found in subsequent encyclicals. Indeed, by the time Vatican II issues its pastoral constitution, Gaudium et spes ("Joy and Hope"), we find the first use of the expression "the principle of subsidiarity" in the body of the text. (See ¶ 86,c.) In Pope Paul VI's follow up review, Octogesima Adveniens, we find further efforts to articulate this new principle starting in ¶ 46. In reviewing Rerum Novarum yet again, Pope John Paul II gave the clearest articulation of the need for the principle of subsidiarity in ¶ 15 of Centessimus Annus as it applies to the role of the state.
During all of this development, the Church continued to focus on the primary good of the individual as being "integral human development", a concept which treats the human person in all of his/her dimensions, including physical, emotional, intellectual, spiritual and social. A policy that does not address all of these dimensions runs the risk of distorting human good. Furthermore, the Church continued to expand on the role of government as "servant leader."
Servant leadership is an essential component of subsidiarity. A servant leader is someone who serves, i.e., places himself/herself at the disposal of those he/she governs, helping them to accomplish their goals (to the extent that those goals are worthy) or (otherwise) explaining why those goals are not worth pursuing. In an ideal situation, both the citizens and their leaders work to advance the common good. A particularly capable servant leader is often able to inspire people to a better developed vision of that common good. Such inspiration is the seed of any genuine reform. That seed, however, must grow and flourish in the population, so that reform may emerge fully developed from that same population. Reform, whether good or bad, generally fails in some fashion when it is imposed from the top.
With the principles of solidarity, subsidiarity, integral human development and the common good in mind, the Compendium goes on to articulate principles of a vibrant and stable economic order, principles which had been developed over the 100 years of reflection beginning with Rerum Novarum.
 The free market cannot be judged apart from the ends that it seeks to accomplish and from the values that it transmits on a societal level. Indeed, the market cannot find in itself the principles for its legitimization; it belongs to the consciences of individuals and to public responsibility to establish a just relationship between means and ends. The individual profit of an economic enterprise, although legitimate, must never become the sole objective. Together with this objective there is another, equally fundamental but of a higher order: social usefulness, which must be brought about not in contrast to but in keeping with the logic of the market. When the free market carries out the important functions mentioned above it becomes a service to the common good and to integral human development. The inversion of the relationship between means and ends, however, can make it degenerate into an inhuman and alienating institution, with uncontrollable repercussions.
How these objectives can be made to balance out in practice is an interesting question. Enterprises such as pornography, prostitution, the sale of highly addictive drugs, elective abortion, etc., tend to be present in all societies even though their operation is ultimately damaging to the social fabric. In some societies they operate freely, perhaps governed only by licensing, inspections, etc., while in other societies, the damaging nature of these enterprises is generally recognized, yet they continue to operate "underground." No society governs itself perfectly.
Societies have come to recognize the need to curtail economic monopolies and collusion, which distort the natural operation of competition in the provision of goods and services according to natural supply/demand logic.
Many, if not most, societies operate with the sufferance of a corrupt collusion between government regulatory practices and regimes and the special privileges of existing economic firms (which tend to exercise such influence due to their size and concentration). Corruption of this nature naturally grows in proportion to the concentration of government regulatory power. Indeed, one way to diagnose a society governed by the principle of supersidiarity (concentration of power in the top level) is to observe the extent of such corrupt combinations, though detection can be problematic when decisions at the top levels are opaque and "whistle blowing" is punished, not encouraged.
Another major indicator of danger in a society is Utopian thinking. The Catechism of the Catholic Church addresses this issue in ¶ 1885: "The principle of subsidiarity is opposed to all forms of collectivism. It sets limits for state intervention. It aims at harmonizing the relationships between individuals and societies. It tends toward the establishment of true international order." Societies that cultivate increasingly collective solutions to all problems, with power to make decisions about those solutions increasingly concentrated at higher levels of governance (such as we find in all socialist and nominally communist countries) ultimately stifle human creativity, autonomy and spirit. Such societies inevitably fail. The tendency all societies have to follow such a path arises when economic power is increasingly concentrated and subsidiarity is abandoned as a governing principle in the false hope that growing governing power concentration will balance economic power concentration. When ideals of equality overshadow those of individual autonomy, responsibility and opportunity, watch out!
Another important indicator of the concentration of power is the tendency of political powers to divide the people along class, ethnicity, religion, gender and other lines, and to foment discontent among minorities,sowing mutual suspicion and rivalries for special privileges, and effectively forestalling the development of grass-roots opposition to corruption at the highest levels.
Societies that tear at the fabric of solidarity by sowing division and mutual resentment do not flourish. Growing infighting increasingly absorbs what creative energy might otherwise be expended in the service of the common good. Indeed, as Jesus said [Matthew 12:25] and as President Abraham Lincoln once observed,
Every kingdom divided against itself will be laid waste, and no town or house divided against itself will stand.
If your country is afflicted with these vices (as mine is), it needs your prayers.