> What I mean is, Christians don't even agree what the word means.

They actually substantially do. There are ~2.2 billion Christians in the world. Of these, about 1.2 billion are Catholic, 0.9 billion are Protestant, and the ~0.2 billion change are Orthodox. Everyone agrees the Catholics are Christian. Pretty much everyone agrees the Orthodox are Christian. Those two groups agree each other are Christian. In Protestantland, things get complicated -- major denominations range from membership in the 85M range to the 40M range, but again, everyone agrees that these major groups are Christian. As the groups get more obscure and more independent, people start to fall back on first principles, but my point is this -- the groups I've named account for something like 98% of the people on the planet who identify as Christian, and they all more or less agree that they all are Christians.

With Mormonism, it is different. Mormons say they are Christians, but Catholics, Protestants, and Orthodox agree they are not. At 14M believers, they are smaller than the smallest of the major denominations, and certainly don't command the sort of numbers such that voting for themselves significantly changes the percentage of Christians in the world who agree they are Christians.

These sorts of things are both communally defined, and reference an objective standard -- and the fact that nobody agrees exactly about what that standard is does not make it subjective.

Sounds complicated and confusing? It's really not. You already are familiar with a community that works this way: science.

Lots of people in the world claim to believe in science, or even to be scientists. Not all of them are. There is an objective standard of behavior, and even if we don't all agree about exactly what it is, we definitely agree that it's objective, and we agree about who meets it the vast, vast majority of the time. The conspiracy crank on the internet says he is a scientist, and they physicist at a major university says he definitely is not. The outsider might just throw up his hands and say, "I'll count whoever claims the term", but to the insider that's very silly. One of those guys really is a scientist, and one really is not.

Organizations help, but are an imperfect standard. Being a professor of hard sciences at a university does not make you a scientist, and being in leadership of a large and well-recognized Christian organization does not make you a Christian, but, well -- these organizations are pretty good at policing themselves, so the folks within them, particularly the farther you go toward the top, are almost always legit.

There are disagreements around the edges. Some will say those folks are doing soft science, and others will say it's not really science. Some will say some obscure denomination has gone too far by rejecting tradition too much, others will say it's fine. But substantially, substantially, the vast majority agree on who is in and out. You will not find a teaching or practice where 30% of Christians think it disqualifies you and 70% think it does not. It's 99% or 1%, almost without exception. (Infant baptism might be the exception that proves the rule here ;) )

If you were an outsider to science, I would say your best bet to determine whether someone was a scientist would be to ask someone from a known safe institution. Find a local geology grad student at a Real University, and ask him if this guy off the internet qualifies. The same test works for Christianity. Find a relatively educated Catholic, Orthodox, or mainline Protestant, and ask them if the person you're wondering about qualifies. That'll give you a pretty accurate answer.

There's a quick and dirty test, too. For science, I would say it is this: does the person you're talking to embrace the doctrine of falsification? They're probably doing science. For Christianity, I would say -- does the person you're wondering about embrace the divinity of Christ? Probably Christian. Not a perfect test, and as an outsider it'll be hard to execute correctly, but it's not a bad place to start.