They were born in mortal combat. Christianity was conceived under Roman occupation of Israel and spread to Rome and its other colonies through dispersed Mesopotamian natives. Islam was conceived under the real possibility of total eradication of Arabic people, unless they could somehow be unified. You got to to admit, it worked. It still does. Of course, Islamic factions did have their differences - and still do - the same as Christian sects did - and still do.The Hebrews developed their religion in the same combative style for the same kind of reason: surrounded by expansive empires and other land-hungry small nations, a jealous, irritable god was pretty much a requirement for survival. They believe it still is. Obviously, if similar religions are practiced by peoples who covet the same real estate, those beliefs are going to be used as a pretext for all-out war. It makes a far more compelling cause than the kind of gentlemen's quarrel the British and French carried on for a few centuries. All kinds of Buddhists, Hindus and Sikhs managed to slaughter one another without recourse to a holy book - that doesn't make their religions any more valid.