The main factor is here undoubtedly Christianity. The Ancient
Greeks may have distinguished themselves from the eastern
Persians, but they did not align themselves any more with the
barbarian tribes in most of Europe at the time. In fact, they at
least appreciated the civilisation of the Achaemenid Persians.
Other factors, most notably the Roman Empire and it successors
the Byzantine and Holy Roman Empires contributed to uniting
Europeans as a single entity. The partitioning of the world in
this sense started after the growth of Christianity really, and
properly after the rise of Islam. In all fairness the "West" is
really synonymous with the European Christian world, which
happens to lie mainly in the west of the Old World. The "East"
refers to the non-Christian, predominantly Muslim, Hindu, and
Chinese/Oriental regions. Africa usually isn't included in this,
due to the lesser role the continent played in world religion
and politics for the most part, and never really posed a threat
to European religion or c