The Archbishop of Canterbury tells us solidarity is at the heart of Christianity. That’s not the Word the Bible uses.
The origins of solidarity in modern politics is somewhat different to that. I attended a Methodist school with a Christian education in RE classes. I was never introduced to the word solidarity in those sessions, and never saw it appear on the pages of the Bible translations we used. At the heart of Christian teaching was the idea of Christian charity, and the modern political versions of it in Christian Aid. The relevant Bible passages were about the rich and powerful helping the poor and needy as an act of charity. They gave them money, jobs, support without expecting anything in return. They did so because it was morally good to share some of their wealth and power, They should not pass by on the other side without helping those in need. The unreformed Catholic Church of the medieval period sold pardons and the promise of eternal life to the rich to sustain clerical incomes and to pass money to the needy. These practices had their supporters and produced an early limited welfare state with hospitals and some support for the poor, but also bred their critics over clerical use of the money. It led to the huge Protestant revolt and the dissolution of the monasteries in Protestant countries. In England it led to a flowering of charitable giving by the newly prosperous landowners and traders that benefitted from the dissolution, leading to many almshouses, and the Elizabethan poor relief system organised by parishes.
Solidarity is a concept from the Union movement. Most famously it became a well known political movement in Poland in the 1980s, seeking the overthrow of authoritarian communism. The idea of solidarity amongst workers is not the same as Christian charity. It is a mutual insurance and assurance scheme. Each Union member pays Union dues. These are used to promote their shared causes, and some of the money is used to help individual members in need of legal assistance or temporary income support because they have hit hard times. The Union member pledges to obey Union rules, and to withdraw his or her labour should the Union by ballot decide on industrial action. The mutual part is based on clear obligations or responsibilities on the Union member, in return for various benefits and the possibility of joint action in need.
The EU took up the idea of solidarity as an important concept in the Treaty of Lisbon and thereafter. The idea of EU solidarity is to tell member states they have to meet their responsibilities to the economic and political union, in return for possible help in their times of need. There is an implied promise of assistance should their state fall on hard times or suffer some natural disaster. That part is a mutual insurance scheme. There is also a mutual assurance scheme that one state threatened in some way would qualify for the support of all in a just cause under the Treaties. The member state has to promise to keep to the rules of the Union, to pay money into the Union coffers, to accept joint action and be willing to come to the assistance of other members in specified circumstances.
The wealthier EU countries led by Germany do not think solidarity requires them to send large sums on a charitable basis to the poorer parts of the Union. Nor does the concept extend to meeting the internationally agreed target of 0.7% for overseas aid. The offer of mutual support can also be selective, as Greece and Cyprus discovered in the Euro crisis. Solidarity leads to a modest scale of regional and social grants at EU level. It is a frequent demand on recalcitrant member states when the EU is seeking to get to a collective agreement, a reason given to make compromises.
Solidarity in the sense of helping the poor is also hedged and often queried by member states. The EU has struggled over the issue of migration and borders in trying to decide how much of an obligation it owes to the poor of the non EU world. It has ended accepting miles of border fence and efforts to deter illegal settlers. Currently the EU wishes to buy up supplies of vaccine for its own citizens, not to help distribute vaccine to the low income countries of the world as the WHO would like. I am not sure this squares with the Archbishop’s view of Christian values.