But quite long in history the Elbe river was considered the east/west divide, Italy would fit neatly in there.
It is true that for a time the Elbe was a boundary between Christian Germans and still-pagan Slavs. But I don't think that ever led to the Elbe being considered the start of "eastern Europe"--except in the Cold War era, when the term was simply used as a synonym for "Soviet-bloc Europe." Even then, some journalists would note that some Warsaw Pact nations were not really in eastern Europe: "Czechoslovakia is the only Communist state in central as apart from eastern Europe, except the German Democratic Republic; its western tip is only 250 miles from the Rhine." John Gunther,
Inside Europe Today (1961), p. 314.
https://archive.org/details/in.ernet.dli.2015.143275/page/n305
The concept "eastern Europe" is simply too new for everything east of the Elbe ever to be considered part of it (except as I noted for Cold War purposes):
"The term “Eastern Europe” suggests a self-contained world of kindred regions. This is a fiction, and to some extent we can reconstruct precisely how it originated. It may be founded on older images of Russia, but it goes back primarily to the French Enlightenment.
"For centuries before that, the only cultural and political axis of significance had been the north-south divide. Following the Renaissance, the “barbaric” regions north of the Alps sought to appropriate the political and cultural legacy of the Roman Empire. This led to the development – in parallel with colonialism – of competing notions of what constituted the center of the “civilized” world.
"The impetus for a shift to a west-east cultural axis came from Paris around the middle of the 18th century, when French Enlightenment thinkers pronounced the orient – which combines the geographical “east” with proximity to the “orient” – to be that region that remained closed to the French or European Enlightenment. In Voltaire, for example, we find the term “Orient de l’Europe”. This refers to an in-between zone that is geographically part of Europe but has yet to benefit from the new philosophy.
"This provided a vector for how the differences within Europe were perceived. In his book
Inventing Eastern Europe (1994), the historian Larry Wolff showed the extent to which, after Peter the Great’s Europeanizing reforms, Russia became a space onto which fantasies of influence could be projected, as if onto a blank surface.
"The east-west axis marked out a descent from the center of enlightened civilization into less and less civilized zones. For travelers, the road to the East,
from its supposed beginnings in Poland, Hungary, or Galicia, [my emphasis--DT] became a voyage into increasingly Asiatic zones. In the process, perceptions were adapted, sometimes in highly fanciful ways, to fit in with the preconceived expectation of encountering a lower level of civilization. Barbarism was now located in the East..."
https://www.unibas.ch/en/Research/U...pt-of-Eastern-Europe-in-past-and-present.html
(Yes, I know that Konrad Adenauer in the 1920's when taking a train to Berlin would supposedly mutter as the train crossed the Elbe bridge "Now we enter Asia." But that was just the product of a Rhineland Catholic's dislike of Protestant and "red" Berlin, and was not meant to be taken seriously.)
In any event, Rome was Catholic long, long before the Slavs were converted from paganism. So even if the once-Wendish areas of central Germany were ever spoken of as "eastern Europe" (which I doubt, outside the Cold War context) it would be infinitely harder to apply that term to the very homeland of the
Roman Catholic Church.