All the sects within Christianity are real to themselves. None are real to each other. None are real to non-believers.
Check out the "No True Scotsman" fallacy.
You're right, though. Because even Christians of one sect consider all those of other sects heathens, our numbers are actually larger than usually reported. Heathens should be considered a larger, more popular group than any other single Christian denomination.
Hmmm...