This should be on your list, if it isn’t already: the spread of Christianity throughout Europe, and from that throughout North & South America.

Christians were a minority sect in the empire until the conversion of Emperor Constantine in the 4th century A.D. With the emperor’s support the church in Rome thrived, even as the empire itself declined. It was sustained in all of the empire’s former territory, including most of Europe; even England.

Without Constantine’s support, Christianity very well might have faded from history long ago.