I say [this] as a modern orthodox Jew who believes in G-d: anyone who believes that "the Rebbe" is moshiach is a heretic.I have heard many similar arguments from frum people, saying that Chabad is not "true Judaism" but rather a messianic cult (see David Berger's book for example). But I don't actually want to talk about that here, mainly because as a non-believer, I don't really care what Halacha says. Rather I want to address the attitude conveyed in the last sentence quoted above. We have a frum person smart enough to recognize the absurdity of Mishichist beliefs, who is seemingly not able to see that many of his beliefs are no less absurd.
Why? Partially because according to Rambam, the Moshiach can't die. But, it's also because this garbage is so stupid, intelligent people such as yourself rightfully reject it.
This is similar to the common occurrence of Christians and Jews mocking the beliefs of Mormons and Scientologists. They will make fun of magic underwear and the galactic overlord Xenu, while themselves believing that God hates pork and that there was a talking snake. There seems to be some kind of cognitive dissonance here. Why can't one group of religious people see that their beliefs are as ridiculous and unfounded as another group's beliefs? My first instinct is to say it is because people get used to their own weird beliefs since they grew up with them, but this does not account for BTs or converts.
Whatever the reason of this phenomenon, wake up! If "their" irrational beliefs are so weird why are "your" irrational beliefs any better? Why can't everyone see that belief in Allah, Jesus, or Hashem is just as ridiculous as belief in Xenu, Rael, or the FSM? If you were born in Salt Lake City instead of Borough Park, don't you think you'd believe in the Book of Mormon just as much as you now believe in the Torah?