Like stated above, Hitler, Pol Pot, Mao, Idi Amin, Stalin - these together blow your statement out of the water
If they weren't dying for a religious cause, they'd be dying for territory, resources (water, oil), control...lots of other things.
The Christian Church is only TWO thousand years old, give or take a couple of few years. You DO know why the Crusades existed, right??? It was Christians REGAINING control of lands that the Muslims conquered in earlier times (from 670 AD to about 1250 AD). The Crusades were several different campaigns that lasted over 400 years, culminating with the Siege of Vienna in 1683. This pushed the Muslim hoardes back to what was left of the Ottoman Empire. A Islamic caliphate that ended after WWI, in 1923.