I think religious groups can agree to differ, but they often think otherwise.
It's difficult for Westerners to understand, religion is a 'background' aspect of our daily lives, but for faiths such as Judaism and Islam, it is woven into the fabric of their daily lives, and teachings (and differencces in them) impact far more for them, than for us.
There is no reason for groups with differeing religious or cultural beliefs to 'unite', the trick is for them to exist side by side in harmony - that's the hard part.