I have not understood why there was any question during the several hundred years before the chorban about the Samaritans. Why would it ever be permitted to rely on anything they did in halacha, whether shechita, matzah or anything else? Would we rely on the Reform or Conservative? Once a sect abandoned Torah She Be'al Peh and adopted the ridiculous regard for Har Grizim, wouldn't they be totally outside of Klal Yisrael?
David Goldman, NYC
I think that Har Grizim only became a problem when they found the image of the pigeon that was Avodah Zarah. For example, if a certain sect of frum Jews would say that they understand that a certain mountain is a holy place because their Rav or Rebbe said it was, this would not put them out of the camp of frum Jews. As long as they perform the Mitzvos properly and only Daven to Hash-m at this "holy place," I don't think anyone would look at this as more than some kind of custom or tradition gone slightly awry. Finding Avodah Zarah there showed that it was a lot more than that, even though only a minority of Kusim used to go there (see Rashi here DH "Matzu"). The Ramban says that they only were officially cut off after most of them were clearly not following the Torah. This is clearly stated in the Yerushalmi.
As far as I know, they did not officially renounce Torah she'Ba'al Peh before the finding of the idol. They just did not happen to know it very well as a group, and only knew certain Mitzvos very well.
All the best,