Hell of an analogy. Rome fell as Christianity ascended in Europe.
This led to Paulus Orosius' Seven Books of History against the Pagans, highlighting all the disasters that had happened in the centuries when the gods were duly worshipped, and St. Augustine's great work The City of God (Charlemagne's favorite book).
Maybe recent poll respondents are just more honest about their views. In prior decades, many of those who would identify themselves as christian in polls were really secular, occasional, or socializing church goers who were far from biblical Christianity.
Exactly. And Gibbon asserted that the rise was one of the causes of Rome's fall (although that claim is disputed).