About the only way you can group all Christians together is that they believe a Christ of some form existed. But that definition is loose enough to include atheists, muslims and others.
Humans have the ability to use context and reason to determine what a person means when they say they are a Christian. 99.9% of the time such a person person means that they are a follower of Jesus Christ and that they believe Jesus Christ to be the son of God.
Of course there will be disagreements on the exact definition, their are disagreements on the exact definition of the word programmer. That doesn't stop it from being a useful word. Any label has exceptions.
But this itself is devoid of meaning. Many people say they follow christ but may have completely different beliefs on what that means. Even believing the son of god is not universal among Christians.
These aren't just blurred lines, it's more like saying a car mechanic, an airline pilot and a web developer are all programmers.
It is nearly universal--universal enough to be useful. If someone says they are a Christian, absent any other knowledge, you can be very confident that they mean that Jesus Christ is the son of God. Obviously not 100% confident, but confident enough to proceed with that assumption in most cases.
I could say that a car mechanic, a pilot, and a web developer are all skilled trades. That doesn't make the term skilled trade meaningless, it just makes it less specific. "Christian" is a very broad classifier and it is about as useful as any other broad classifier. It lacks specifics, but language is recursive--I can add as many extra identifiers as I like, e.g., Protestant Christian, Roman Catholic Christian, and Orthodox Christian.
At the end of the day even though we might call ourselves different types of Christians based on the traditions and non-essentials that we hold to, there are still Christians from every tribe, tongue, and nation that are united by Christ... and Christian is really only the word I can think of that makes sense.