For example, Christianity teaches that only proper Christians get to go to heaven, and only Christianity teaches that, at least that's what I've been told growing up, and it doesn't matter how good of a person you are, if you're not a proper Christian you do not get to go.
That is only what a particular Christian sect will tell you, it isn't what the Bible says.
Tram law writes:
And for me, if God exists, it is only God who gets to determine who is a proper Christian.
I'm quite sure God isn't concerned about the definition of words. God is concerned about the condition of your heart.