One definition of a cult I've liked is "A cult is a group you can't leave with your dignity intact". You can really get a good understanding if something is a cult by the way they talk about their ex members. A healthy organization will pretty much just say "Sad to see you go, come back anytime". Unhealthy organizations spend a fair amount of time warning members about how how dangerous/bad/sinful/sad/miserable their ex members are.

And yes, I'd call several mainstream religions cults because of this. I'd not argue that something isn't a cult if they don't fit this definition, but a very large number of the most damaging cults do have this behavior.