I saw that remark on Facebook not long ago, and it struck me as true. The reference is to John Courtney Murray, S.J. I have not read Murray, but my understanding is that he articulated the idea that American institutions, particularly religious freedom, and Catholicism are fundamentally compatible. He is said to have been influential in Vatican II's declaration on religious liberty, Dignitatis Humanae. He seems to have been a big influence on First Things magazine and the whole attempt of politically conservative Catholics to influence American politics. The basic idea, as I understand it, was that a common foundation on "truths [held] to be self-evident," amounting to what Catholic theology understands as natural law, was sufficient to allow church and state to co-exist in reasonable harmony.
Well, it's not working out. It seemed to be for a while, but it's collapsing before our eyes. Whether it's intellectually and historically justifiable or not, the grounding of the American constitution in something like natural law--the "Laws of Nature and of Nature's God" so admired by the Founders--has been discarded by the authorities whose understanding of the Constitution is law. The situation we have now is not of religion and the secular authority coexisting in mutual respect, but of a conflict between two incompatible religions--a loose collection of various forms of Christianity, on the one hand, and materialist progressivism on the other. The latter is also the faith of the secular authority, which is seeking to impose it in every situation where the two come into conflict in the realm of practice.
Set aside the theoretical arguments for a moment, it is, I'm afraid, simply a practical effect of human nature that societies require some sort of commonly-held metaphysic. The rationalism of the 18th century, implicitly including some Christian axioms, left something of a vacuum there, a gentleman's agreement not to press the issue too far, and its more vigorous and militant descendant, having cast off the remaining Christian elements, is now filling the vacuum.
I've never been able to get enthusiastic about the idea of a Catholic (or any sort of Christian) confessional state, the main reason being that the inevitable corruption that goes along with political power ends up discrediting the Gospel itself. But I'm wondering now if that's the only alternative to a political order in which Christianity is at best marginalized and at worst persecuted. Maybe the modern conception of more or less unlimited religious freedom within a single polity was never realistic. At any rate it seems workable only within certain limits, limits which the Murray project perhaps thought reliable, but which have proved not to be.
Perhaps Archbishop Lefebvre was right all along about Dignitatis Humanae.