It would need an introduction of Eurasian livestock during Classical times (1000 BC to 250 AD) for there to be enough time to get an enormous China-size population. I dunno if a wayward Phoenician or Roman ship bringing those to the New World is ASB or not.
The notion that Christianity weakened the Empire is the creation of Ed Gibbon and is based on Enlightenment-Era secular polemics, not fact. Unfortunately this bad history was re-popularized by Carl Sagan's ignorant rant about Christians burning down the Library of Alexandria in Cosmos. :rolleyes: