John, perhaps the question here is "what is a 'true believer?'" For one example, in the Catholic religion, most American Catholics have long since adopted the use of birth control other than the rhythm method, but would still call themselves Catholic. Every doctrinal point does not define them, but at the core, they consider themselves Catholic. Rome might wish they were more devout, but basically they are happy to have the flock show up at Mass once in awhile and throw a buck in the plate. The same goes for most other religions. Not every Mormon wife wears religious undergarments, not every Baptist believes every non-Christian goes to hell, and so on.
TWI, on the other hand, became much more insistent that their followers adopt every bit of the dogma, even when the articles of faith seemed to change from month to month during the Craig years. (Original sin, tithing, allegiance to the MOG, and debt come to mind.) Worse, the organizational structure seemed to support ratting out anyone who did not conform, and those who were ratted on were kicked out, sometimes without so much as a shred of evidence.
During the Weirwille years, I knew of plenty of believers who didn't buy every piece of the doctrine. Heck, I didn't, either. But I was willing (for a time) to stay with the organization, because overall, where else was I going to go where anything close to this doctrine was preached? So I and many others stayed, hoping to make changes from within, or to reason with people one at a time.
So the basic difference, socially, between TWI and other religions is one of tolerance. I find it interesting that the "kinder, gentler" TWI kicked in when it was clear that a rigid stance threatened to destroy the organization. Can't have that -- too many people on the payroll.