I think if religious and ethnic identities are considered to be vague, and if it is that specific quality that is their drawback, then surely that means abstract creeds and values are even more so? How can that be compared to something tangible and material? I’ll refer to the Sinicization of Tibet for why it matters. China and Tibet might be similar, but they’re still, quite different. The cultural uniformity that you would require in a global sense can only be achieved using China’s methods, unless you took a long, long term approach.
If it already exists, then sure a common creed can work.
Now that is a very extreme example, an extreme one, but I think it proves a point.The changes, evident since the incorporation of Tibet into the People's Republic of China in 1950–51, have been facilitated by a range of economic, social, cultural and political reforms introduced to Tibet by the Chinese government. Critics cite the government-sponsored migration of large numbers of Han Chinese into the Tibet Autonomous Region as a major component of sinicization.
The government in exile says that the policies intend to make Tibet an integral part of China and control desire for Tibetan self-determination.
The Chinese government maintains that its policies have benefited Tibet, and cultural and social changes are consequences of modernization. According to the government, Tibet's economy has expanded; improved services and infrastructure have improved the quality of life of Tibetans, and the Tibetan language and culture have been protected.
I’d like to reference Obama’s 2008 speech, where he talks of uniting ‘many colours and creeds’. Now let’s assume he really means that, literally (probably not but anyway) What do you use to unite if colour (specifically not a great choice really) and creed (better) are ruled out?
It’s interesting you mention about ‘some medieval army’. I don’t think it’s as arbitrary as you think. These things heavily define the present and future. I think there’s a big difference in different countries when it comes to attitudes to past armies and wars, and it varies person to person. An Appalachian Presbyterian will likely tell you all about his ancestors in the Independence War, while perhaps the average Californian might not, even though California is probably the region that was influenced more drastically, culturally in the long run (American manifest destiny, etc). You’ve got the Boyne, Hastings, Siege of Constantinople, and so on.