I’d like to bring up an important point, which has very likely been misunderstood by our Lubavicher friends.
What would happen if a rebbe said explicitly, “I am the Mashiach”? What should his chossid think? The answer is that he should definitely believe him! However…
1. He should not say that the rebbe has already fit the criteria of the Rambam (unless he has – but no one in the past millenium has).
2. He should not try to (mis)interpret the Rambam, or any other source, to make it fit the rebbe’s current status.
3. And he should not call anyone who doesn’t accept his rebbe’s statement a kofer or an apikorus. (I don’t believe that anyone on this thread has, but there are a number of prominent Lubavichers who have publicly stated as much in the past. I could cite at least one name, but I’d rather not.)
Now a corollary question: could there be any stage when that chossid should reject his Rebbe’s statement, and possibly even find a different Rebbe? I believe the answer is yes – remember, I am a card-carrying member of a chassidus that you are probably aware of – but it would have to be only when the rebbe crossed numerous red lines.
For example (I bet you can guess at least some of what’s coming):
1. If the rebbe was a baal machlokes with many other recognized gedolei Yisroel, or if he consistently denigrated them.
2. If the rebbe himself came up with ludicrous proofs that his way was the best way for all Yidden. An example of ludicrous proofs could be the LR’s explaining how not sleeping in the sukkah shows a higher level of halacha because one is mitzta’er that he can fall asleep due to his hiskashrus. (We’ve been through this before a number of times.)
3. If the rebbe consistently came up with heretofore unheard-of concepts to self-promote, e.g. dor shevi’I, Nassi Hador, the return of nevuah, 770 being Beis Mashiach. (There are more.)
That, at least, is the way I look at it.