If Scientology was the main religion all the way back to WhateverBC, and some guy proposed Christianity, we'd all think he was a nut, wouldn't we?
Probably not in comparison, because on the whole a religion which doesn't necessarily demand specifics (or at least not the fundamentalist branches) leaves a lot more open to interpretation than the Scientologist mythos. Much less the fact that the details of Scientology's philosophy would not have been developed in the same manner in that time period.
Especially when you consider that Christianity is largely based on historical (arguing this is largely irrelevant to my point, since the mere premise alters the fundamental nature of the religion regardless of its veracity) evidence and sightings, testimony considered credulous at the time. Compare that to Scientology which is much more "I saw this all in a dream", and you've got a very different beast.
Not that this is an endorsement of Christianity as a whole, but in terms of a scale of credulity, Scientology is waaaay further off the end than most contemporary mainstream religions.
