[Kolokotronis]

"...what makes τὴν οἰκουμένην τὴν μέλλουσαν specific to Roman Empire other than the historical usage of the word?"

Nothing more, really, than the times within which the words were written. The "οἰκουμένe" originally meant the inhabited world but it came to mean the civilized world which was seen as being co-extensive with the Roman Empire. Later and today, when used by The Church, it means the commonwealth of the Orthodox Christian people of the world (which down deep also is seen as definitional of being truly civilized in a Roman/non-barbarian sense).

[kosta50]

Nothing more, really, than the times within which the words were written. The "οἰκουμένe" originally meant the inhabited world but it came to mean the civilized world which was seen as being co-extensive with the Roman Empire. Later and today, when used by The Church, it means the commonwealth of the Orthodox Christian people...

In other words, some "sola scripturalist," reading the Bible "cold," and in Englsh, as

"It is not to angels that he has subjected the world to come, about which we are speaking." [Heb 2:5]

would not be able to understand that contextual difference?