It is not a harmless shared delusion. That is to say that it is not harmless. There are Christians that are harmless but the religion itself, if taken as directed, is every bit as harmful to the body politic as cigarettes are to an individual.
This is unfortunate because there is much to like about Christianity but unless you are very similar to cafeteria Catholics(and at that point how do you straightfacedly consider yourself Christian) then there is at least as much to be appalled at.