Christians: Why is there so much concern with who is a real Christian?

Just before we get started, I know these conversations can be a bit tense sometimes, so let me say this: I’m not looking to bash Christians or tell them why they’re wrong, and I’d appreciate it if others followed suit.

I have noticed a tendency among (some?) Christians to state “Oh, but so-and-so isn’t a real Christian, because x, y, and z”. This seems to be a fairly large conversation, often in regards to things like Mormanism, drunkenness, homosexuality, etc. While I understand this in regards to family members and other close loved ones, because presumably only those who are true/real Christians can get into Heaven (right?), I don’t understand this conversation when it’s about people you aren’t that close to, and don’t really care about, especially strangers on the internet. So, I’m wondering what the reason is for this conversation, this fight over who is and isn’t a real Christian, why this is a discussion not only worth having, but having worth having quite a lot and rather fiercely. Any insight you have would be appreciated.