To be fair, do not some Roman Catholics do the same?
They are of this rite, or that rite, or, most commonly, are Cafeteria Catholics or CINOs. They become of Father this-and-so or This Holy Order of Nuns. Many official nuns and priests and monks actively oppose official church doctrines - and they aren’t excuseable as “just members.”
I don’t think the charge of sectarianism lies at the feet of Protestants alone. The Roman Catholics are NOT all marching to the same beat.
You might say, well, then they are not true Roman Catholics. I can see that. But then I’d say, all those “Protestants” who deny basic gospel truths are not true Protestants, either. That knife cuts both ways.
Great point. Thanks.