Centuries ago, people were more isolated and clustered together in like-minded groups. For example, whole countries and even continents followed just one religion, or spoke one language, hardly ever setting eyes on anyone who followed another or spoke other languages.
Though we may not really recognize it today, significant population shifts and migrations often led to conflict and political upheaval. It seems to be a basic human instinct to fear the unknown and even loathe it - but with knowledge and understanding come tolerance and enlightenment, but to get to that stage people first have to get past their fear and loathing.
The idea that all people can and should live together in peace is not really a new one -
but yet in practice, it is very young indeed.