Those situations mentioned are not exactly analagous. Catholics in Reformation England had to hold out for what, a century and a quarter? During that time, they had the benefit of literacy, varying degrees of sympathy among the nation’s elite (including the occaisional monarch) and ready physical access to an adjoining continent which was overwhelmingly Catholic in nature. I’m not saying it was easy to hang on in those times, and plenty died for their faith, but they didn’t face the problem of forgetting “how” to be Catholic. Christians in the Soviet Union certainly didn’t have an easy slog of it either, but their dark time lasted all of 75 years.
Pagans in England, just as one example, had to try to reconstruct and start over after 13 CENTURIES of effective absense. During that time, the entire cultural, ethnic and political landscape was swept away by countless invasions. They had essentially no written records to turn to, and even oral histories were disrupted as the Industrial Revolution forced people to move from rural areas where they may have had family roots for 500 years.
The pope’s recent visit to the Czech Republic illustrates how fragile religious identity can be in the storms of political change. With only a generation or two of communist influence removing the church’s influence from public life, the country went from one of the more devout Catholic nations to one that is mostly indifferent to the faith. Regardless of the merit of the religion in question, it’s ultimately a human endeavor subject to the laws of momentum.
The other thing to keep in mind is that no one pagan tradition became the religion of empire the way Christianity did, at least post-Roman times. When genuine conversion, or bribery of a chieftan, or threat of death extinguished the flame in one area, it’s not like they could send to their version of Rome for more missionaries, backup copies of their liturgy, etc.
In truth, SOME pagan traditions did surivive time more or less intact. Not many, to be sure, but in the Baltic region, which was never fully Christianized and only somewhat so in fairly modern times, there are traditions which are considered fairly authentic. Even in England and elsewhere, bits of the old religions survived in fragments in the cultural memory, in what would be called folklore, folk magick etc. Many elements of paganism also survived WITHIN Christianity. Conquerors often get assimilated by the cultures they take over, at least as much as they impose their culture on the conquered.
I don’t know how much we should read into the validity of a religion based on its ebbs and flows of practice or political capital. From AD 70 (or before) to the 1960s, really, the history of Judaism was more trough than peak, yet I don’t hear many at least on this forum willing to conclude that it’s fallen from God’s favor and should die.