I’ve wondered about this myself, and the answer lays in the absolutist nature of religions in general who believe you are either right about god or going to hell.
there is no middle ground for such a belief system. Even though I believe it is more the teachings of god that are important than the “nature of god” for example some believe that god is omnipotent and omnipresent. he knows everything and is everywhere. others believe that for this to happen god must exist in a transient state. still others believe he knows everything but is a physical being. while in theory these ideas can not both be true, both sides of the arguement believe if you dont know the truth you dont understand god and will therefore go to hell.. and fundimentaly someone has to be wrong here.
and then you get into the nature of salvation or how your soul is saved which is just not something Im going to type out here because its long and involved and is at the root of many religious confrontations. but those who would profess to believe the bible or the quran, and would seek to engage in such arguements need to remember that the second and great biblical commandment is to “love thy neighbor as thy self” the quran too says to teach the infidel, it doesnt say kill him. but there are those who would interpret it differently. hope that helps.