would there be peace if there was only one religion

I don't really believe religion causes the MOST of the war. There are other underlying factors in it. Like money. There will never be peace because if there are so many poor countries and then a select few hold the whole world's wealth...that is not far. It causes conflict and conflict causes war.

No, i think there would still be conflict. Especially since there are often more than one faction in each religion who fight. For example Catholics and Protestants in Ireland.