He should leave quietly, and convert to Presbyterianism. A public screed calling the ecclesiology he just left "sin" is inappropriate.
Why, because it might offend someone? Actually, Christendom needs more men willing to state openly and publicly what they believe, and point out the errors that have compromised and emasculated the Church in America. We need men who will call sin what it is.
I think the baptists could make a pretty good defense of congregationalism if given half a chance. Therefore, it's another of those items that has a few competing views.
He is being too hard on congregationalists/independents.
Misunderstanding doctrine is hardly sin.
You are right. Were I to go from premil to amil, should I call premil a "sin?"
Is there a command out there someplace that says, "Thou Shalt be amil." or "Thou shalt be presbyterian?"
I don't think so.