Gideon Strauss was kind enough to translate a key section from the CDA's 1990 research report, Publieke gerechtigheid: Een christen-democratische visie op de rol van de overheid in de samenleving (Public justice: a Christian democratic vision of the role of government in society):
The principle of sphere sovereignty and the idea of subsidiarity can probably not be identified with each other. They give expression to distinct values and it is meaningless to try and hide the differences under the table. But, as has been said before, it is not necessary to identify sphere sovereignty and subsidiarity in the effort to arrive at a consistent vision of government. Each complements and corrects the other. . . . For that reason the CDA has in its very brief history been able to achieve a close, authentic fusion of principles with the common denominator of the responsible society. The idea of subsidiarity borne by Catholic social teaching and the idea of sphere sovereignty current among protestants has been woven together into a view of the person, society, and state in which both dimensions are on a sound footing. The fusion of the two traditions had resulted in a single political philosophy that strives to develop a correct relationship between personal, social, and political responsibilities, taking into account the distinctive value of each of the terrains of life, as these have come to flourish in society (pp. 133-4).
Two decades ago I began writing a dissertation on precisely this topic, comparing the political theories of Herman Dooyeweerd and Yves R. Simon with respect to differentiated authority or responsibility. At the time I was much impressed by the seeming practical convergence between the two concepts in support of what is nowadays known as civil society. My tendency then was to tone down the differences between the two. Since then, however, it's become increasingly apparent to me that subsidiarity is really a federal principle devolving authority to a subordinate body within a larger, more encompassing structure. It is embodied in the tenth amendment of the US Constitution and in the European Union's Maastricht Treaty of 1992. It characterizes the relationship between two entities of the same intrinsic nature. Subsidiarity is less effective in enabling us to differentiate between entities characterized by distinctive structural principles.
This is where Kuyper's sphere sovereignty, particularly as modified and systematized by Dooyeweerd, constitutes an advance over subsidiarity. It enables us to differentiate among different kinds of things without having to locate them within some sort of ontic hierarchy. We need not try arbitrarily to assign higher and lower status to institutional church, state, school, business enterprise or labour union in order to make subsidiarity work. Subsidiarity thus applies well to, say, federal, provincial and municipal governments; sphere sovereignty, or differentiated responsibility, properly applies to, say, government and family.
One wonders whether the CDA is still able to affirm the distinction between subsidiarity and sphere sovereignty. A decade after the merger of the three christian parties, it was still able to do so. Another decade and a half have passed, and much has undoubtedly changed in the Netherlands. Given the huge tide of secularism that swept over the country after the end of the Second World War, it may be that Dutch Christians have enough on their hands to maintain any sort of public witness to their faith at all. Ensuring that the public understands the differences between subsidiarity and sphere sovereignty may seem to some like straining at gnats. Yet much is at stake here, namely, a recognition of diversity in God's world, as opposed to the monistic reductionisms of the various secular ideologies.
Those interested in a more detailed discussion of this issue would do well to read chapter 8 of my book.