I dont have any mal-intentions, but just wanted to raise a counter-point.
Wouldn't it be wrong to say that the Christian world (the West) was ever united or they constituted one combined entity at the expense of the Muslims? I mean the Russians are Christians but never sided with the Europeans and the Americans. The Christian-Capitalist US is still having tiffs with the Christian -Socialist Cuba and Venezuela.
What I am trying to say is that religions never was a unifying phenomenon for people with different ethnicities. When Babur invaded India he fought with Ibrahim Lodhi of the Delhi sultanate. Tipu Sultan fought with both the Nizams and the British. Hitler, a devout Catholic waged war with the whole predominantly Christian western world.
I'd say ethnicity and culture are the main deciding factors.
Throughout history people of same faiths have fought with each other simply because religion fails to act as a unifying force. May be that's possible according to your and Zaid Hamid's views, but from what history and the present suggest, there will forever be factors beyond the scope of religion that will unite or divide us.
You have to go back, way back to see the whole game. The British at that time represented Christian empire. It wasnt said straight to the face, but every sane person knows, how they brought their culture and influenced it into other people culture, language and religion. I can give you 10 pages history lesson on British grand plan to subjugate and divide the world population.
I wouldnt say ethnicity or culture is the deciding factor because lets look at history. The Nazis believe in Nationalistic Socialist System. A party that may be composed of different ethnic people but still belonged to the party. Like Italians were supporters of Nazis.
The Communists of Russia, again different ethnicities were under the umbrella of Communism. Show me any where that a huge empire was based only on ethnicity. Heck, even British empire wasnet only one culture, language or ethnicity.