Christian Democracy is the most electorally successful political tradition of postwar Europe and the least theorized, because it was built by people governing and not by people writing.
Its doctrinal source is Catholic social teaching, above all Rerum Novarum (1891), which condemned both socialism and unrestrained capitalism and asserted a right to a living wage, to property and to association. Two concepts do the tradition's work. Subsidiarity holds that decisions should be taken at the lowest competent level, with higher bodies assisting lower ones instead of absorbing them, which limits the state without leaving the individual alone against the market. Solidarity holds that society is a body of mutually obligated persons and not a market of individuals, which licenses extensive welfare provision on grounds no liberal individualism supplies.
Maritain gave it a philosophy. Writing after the fall of France and the collaboration of much of the Catholic right, he argued that a Christian could and must embrace democracy and human rights, and distinguished the person, with an inviolable spiritual destiny, from the individual as a unit of society. He shaped the drafting of the Universal Declaration of Human Rights.
The politicians made it a governing order. De Gasperi in Italy, Adenauer in Germany and Schuman in France rebuilt shattered states on it, and the European Coal and Steel Community was substantially their project: Catholic, cross-border, and designed to make war between France and Germany materially impossible. In the Netherlands, Kuyper's Calvinist doctrine of sphere sovereignty had reached a strikingly similar structure from a Protestant direction.
Its long decline tracks European secularization, and its remnants are visible in the EU's founding vocabulary.