The most dangerous and unfortunately contagious of beliefs are those that believe that if someone doesn't believe the way you do they're damned to hell, especially the almost amoral disposition of evangelical Christianity, which puts belief over everything else. They outright state that a good person will go to hell if they don't believe in God and find no qualms with the such reasoning - that's not only morally bankrupt, it's downright DANGEROUS.
Why do you Islam and Christianity have gotten so large, while the parent religion, Judaism, has stayed almost constant? Purely belief driven religions are a virus.