Christianity (and Islam after it) has the unique characteristic of being the first to claim that it was both universal (applied to all of humanity, not just one's tribe/city/language/culture) and that it was exclusively true -- no other belief systems were considered real. Even Judaism allowed for difference between Gentiles and Jews. Christianity admits no other belief systems beside itself. For all of humanity.
The 'pagan' Romans were astounded at the intolerance and dogma of the Christians. They were used to absorbing other ethnicities pantheons into their own. While they had all sorts of chauvinistic and tribalistic beliefs, they never believed -- until Christianity -- that they had the answers for all human kind.
Christianity is the origin of 'religion' as we think of it now. It has altered the lens of how we perceive all belief systems, so we accept as normal the concept of an all-encompassing totalizing system of transcendent explanation.
Whether you think its explanations are right or not, this is a unique property historically. And Islam inherited this philosophy.
Even more profound is the extreme intolerance that Christianity fostered _within itself_. Many thousands lost their _lives_ for interpreting the some aspect the Trinity, or the relationship of the Father to the Son differently from what became accepted as official dogma. Very technical language and very specific interpretations of this language controlled the fate of nations and the lives of millions.