I think it was because of the weather in winter the Indians would migrate. I think that's the answer.
Based on the map I've attached, I'd say that Christianity spread to
most of the Roman Empire by 476 AD. If you look at the map, you will see that before 325, there were some places in Spain and Italy mostly where Christianity existed, however, after that year, it only continued spreading throughout Europe, which mostly belonged to the Roman Empire at the time.
North was the region the United States were most major cities located by the 1850's.