[Political Interest]

What does the term 'east of the Elbe' mean in the context of the Middle Ages? Was it the border between the Christian "West" and the non-Christian or non-Latin Christian "East"?

We know that Europe was Christianised in gradual stages. It did not happen overnight.

Would places which were formerly considered "eastern" over time have become "western" through their acceptance of Catholicism and Roman Latin Christian civilisation?

For example, Poland and Lithuania were at one point not part of Christendom. However they later accepted Catholicism.