So if you're a Protestant, and a Catholic says you're not a proper christian because difference in religious dogma who's right? What about Jews for Jesus? Is a Christian defined by his faith or his acts?

These things are all up for debate.

To attack your metaphor: Is he a true Scotsman when both his parents are scottish and he was born just a wee bit over the border?

Well, if you disagree with the one central dogma of christianism, it's pretty much the case that you are not christian. It's not the case that your scottish was born just over the border, it's that his family is from Madagascar and he was born there.

Otherwise, what is giving that group of people a name even good for? It's like somebody claiming to be democrat, as long as all the law is written by a single person, or marxist, as long as commerce and private ownership are preserved.

But my point is exactly that: the "central dogma" doesn't exist. Is it essential dogma that gays will not get into heaven? Yes, and also no, depending on the denomination.

When you get a billion people creating hundreds(thousands?) of factions, there are no unifying ideas that bind everyone. I mean the council of Nicea had to happen for exactly this reason.

"Thou shall not kill/murder" is a commandment, and yet the crusades were allowed to happen because of the interpretation of what "kill/murder" means.

Christianity is not a monolith, and the book can be used to justify all sorts of things that other people would feel goes entirely against the core of the faith.

There's no true Scotsmen here because Scotland is a state of mind and not anything absolute.