The hard boundary I see is belief in Jesus Christ.Sorry, but Christianity isn't just some nebulous nothing that you tack any idea or belief to. There's variances but then there's hard boundaries. It's like "Identity Christians" who are white supremacists and think non-white people have no souls and will be destroyed by God one day. Yes, I'm going to say they're not Christians or getting such things from Christianity. That's an extreme example, but proves my point.
That results in a vast spectrum, with some awful
variants. No True Scotsman doesn't exclude them.