What exactly is the prohibition of believing Jesus is the messiah?

I am Jewish. I’ve been on kollel for a good number of years and finished most of my semicha. I’ll say it up front so there’s no misconceptions about where I’m coming from: Jesus is not the messiah. Or even a prophet. He’s dead, not coming back, and in my view, is liable for being madiah if not a zaken mamre, as well. But, what precisely is the prohibition of someone believing Jesus is just the messiah. Suppose someone clearly rejects that he’s God, does not pray to him (no melitza, no beliefs of trinitarianism or any kind of shittuf, etc), he rejects the Pauline doctrines of reforming the law and what not, and basically keeps everything as per what rabbinic Judaism prescribes. Would this be assur? If so, what would be the actual prohibition being violated here? Aside from self delusion, what precisely is going on here?

I mean, they’re not denying moshiach will come or that he even exists. He’s just got the wrong guy and is confused about it. Halachically, what would be the problem. I prefer to see sources that explicitly address this, if possible (teshuvot, maamarim, etc). What are the different takes on this specific aspect of it?