n0mesora:
I don’t want to get pulled back into the Chabad debate, but what you’re saying completely transcends it. If you want to say Chabad’s beliefs are fine and kosher, then go ahead, but you’re actually claiming that the halachah has no concept of kefira whatsoever. Other than being completely untrue, this line of reasoning should offend both sides of this thread.
Someone absolutely can be pasul from aidus on the basis of beliefs alone if they make them public. Would you eat the food of Jew for Jesus as long as he otherwise kept halacha? Judaism doesn’t care about belief according to you, right, so why not? What about an openly proud atheist who happens to keep halacha out of a sense of tradition (like the original idea of Reconstructionism)?
We don’t have to put people under a microscope and try to figure out their beliefs. It sounds like someone probably told you this at one point and you erroneously assumed it to mean that people can believe whatever they want and it doesn’t matter.