That's between them and God as long as they don't try to teach their error to the public. Then I would have to counter with scripture just like we do with the error of the Catholic religion here. If they persist in their error they surely wouldn't be allowed to meet with us.
So it is your way or the highway. I get it.
Christ wants us to be one, not many.
“I appeal to you, brothers, in the name of our Lord Jesus Christ, that all of you agree with one another so that there may be no divisions among you and that you may be perfectly united in mind and thought. My brothers, some from Chloe's household have informed me that there are quarrels among you. What I mean is this: One of you says, "I follow Paul"; another, "I follow Apollos"; another, "I follow Cephas"; still another, "I follow Christ." Is Christ divided? Was Paul crucified for you? Were you baptized into the name of Paul? I am thankful that I did not baptize any of you except Crispus and Gaius, so no one can say that you were baptized into my name. (Yes, I also baptized the household of Stephanas; beyond that, I don't remember if I baptized anyone else.) For Christ did not send me to baptize, but to preach the gospel—not with words of human wisdom, lest the cross of Christ be emptied of its power. 1Cor 1:10-17