This is a partial rebuttal to Nathan Smith's argument that "Christianity is the death of the nation." We have great real-life conversations on this subject, but perhaps there is some benefit in debating some of this out in public.
Firstly, I will agree that he is correct in that Christianity is a religion unconstrained by geography. Burying the ancestors under your house and setting out a meal for them while tending the eternal flame in the hearth did bind people to the land more than Christianity does. However, it seems to me strange to claim that paganism was completely geographically rooted. Mass migrations of people (we call these "invasions") were common in pagan times. They did start new families and new nations. That continued under Christianity.
Every Christian Father speaks freely about our duties to family, tribe, kinsmen, and nation with ease, while recognising that sometimes the Church supersedes these duties. More often our duties to family supersede that of the Church. You have a greater duty to provide for your parents than giving to the Church, to use an example directly from the New Testament. Our natural bonds and duties are not dissolved by the Gospel. God has established the times and boundaries of every nation and he has multiplied languages and spread us across the world. Having your country run by foreigners, women and children is a biblical curse.
Christian nations have existed for a long time, at least since Emperor Constantine in the 4th century. Christianity eventually fully displaces the pagans in Europe and we can identify a clear uninterrupted span of 1,500 years of Christian nations in Europe, with some variance depending on definitions. During the Reformation stronger senses of national identity were developed to counter the "globalism" of Rome, if you will excuse my anachronism.
Peacetime mass migration is a relatively new phenomenon. New Zealand's migration inflows before 1980 are basically zero, by 1990 they're at 20,000 a year, and it isn't until the 21st century that they really ramp up to the influx of 200,000 we regularly see today. By the 1980s Christian dominance over New Zealand society was gone. No-fault divorce, abortion, and Sunday trading are all legalised. What's left of institutional Christianity exists only in name and heritage. I don't know the percentages for New Zealand, but in America 90% of "white evangelicals" are voting for Trump's mass deportation policy right now.
Only under post-war liberalism does mass migration become a problem and universalist theology is elevated to the fore. A feature of Christianity is that it does not specify a system of government or a culture that people must adopt. Christianity does not care for democracy or autocracy, monarchy or republic, oligarchy or plutocracy. Only in the 20th and 21st century do we fully see Christianity employed in the context of "fighting for democracy". We give our enemies democracy, liberal values, Hollywood, and all manner of vices from industries that are not populated with people of Christian faith. When is the last time Christianity was made the state religion of a vanquished foe? We do impose a new religion on the defeated, but it wears the cloak of ideology and universal values that are at best derivative of, and at worst in complete contradiction, with historic Christianity.
The claim must then be that Christianity's universalism took a millennium and a half to slowly decay and deracinate the European peoples after they stopped worshipping their ancestors at the eternal flame in hearth and home. Is this a feature of Christianity, a ticking-time bomb that happened to go off during our lifetimes, or is this a parasitical ideology that would have latched itself into any society and religion, even a pagan one?
Strangely this progressive view is one that is seriously accepted by many post-20th century Christians. Even non-Christian historians like Tom Holland in (his otherwise excellent book) Dominion embraces this view of Christian progress marching on through time. This is the "Whig view of history" and the core tenet of "progressivism" as an ideology: we are always getting better and more enlightened. It took 2,000 years for Christians to discover the true meaning of Christianity. The true teachings of Jesus happen to align perfectly with 20th century liberalism! What a wonderful surprise!
To put it bluntly, this is subversion and exploitation. Any religion could be exploited in this way to create rootless people. ("Carry the ashes of your venerated ancestors with you, bro.") The wrong political theology is popular only because it is the only socially acceptable form of theology. It is popular because it is inoffensive and not because it is a true representation of Christianity.
I'm out of space for today, but I would place the blame for rootlessness and mass migration on a combination of technological advances, rapid urbanisation, the wrong lessons learned from World War 2, deliberate revenge by people carrying historic grievances, and darker spiritual forces. I would even blame the death of Christianity for the death of the nation.
Christianity has the tools needed to defend the nation. We just need a bit of time to recover them fully from the repositories of our great-grandfathers, taken from us by the post-war consensus.