The Pope gets to decide what constitutes a member of his church. You are or are not a member at his sole discretion, or the discretion of his responsible deputies.
I'm against excommunication. They shouldn't be forced out of the church. Instead, if they refuse to recant of heresy, then they ought to be burned at the stake.
Excommunication is impolite. You have determined them to be a liar. Why are you trying to force them on someone else? "I found some shit in my yard, so I threw it into the neighbour's yard." On the flip side: do liars deserve to live? They do not. Do they deserve a peaceful, merciful death? They do not.
Leaving voluntarily should be allowed, or rather, encouraged. If someone wants to profess heresy they're more than welcome to do so as long as they leave first. Your believers should be working to earn their place (so, the opposite of Catholicism) rather than entitled to sinecure. The deal you offer should be worth this effort. You encourage them to leave precisely because, if that works, you deserve to have all your followers leave.
Sectarianism is good. The more sectarian, the better. If having more sects is a problem, the sects aren't the problem.
P.S. Humility means accepting that you might have screwed up.* If you have screwed up, at least don't try to drag everyone else down with you. Humility also means believing it's possible that others can tell if you've screwed up or not.
Or rather, Hanlon's razor is normally false. They know damn fine they're lying to you, which is why they don't want you to leave. They know that if you left, you would rapidly find out first hand they were giving bad advice. Anyone institution which opposes functional humility isn't blinded by pride; they're fully aware they're evil.
P.P.S. *So, the opposite of Christianity. Using the term Satanism for Abrahamism is funny because it's literally true.