If you choose to be called a Christian, your choices are a bit narrower.
The term Christian, plain and simple, means "follower of Christ." That means you follow the teachings of Christ. You read his words, you do what he says.
Not just some of it. All of it.
So when Christ says:
I am the way and the truth and the life. No one comes to the Father except through me. John 14:6
That means, as a Christian, you believe that. Christ said it. As a follower of Christ, that's part of your doctrine.
You can't pick and choose.
If I decided to piece together my own belief system—take a little from here, a little from there, listen to some things Christ said and not others—I would cease to be a Christian. I'd become a Rachelian. I'd become a follower of whatever I wanted to believe. Whatever sounded good to me. Or easy. Or fun.
Rachelianity is not Christianity. Clearly. Even though I've got a Rachelmas to go with it.
But I digress.
Christianity has somehow become this broad idea that encompasses whatever you want. You can't say that Christ died for your sins and in the next breath say that he's not the only way to the Father. It's all or nothing. You either take everything that Christ said and did, or you essentially take none of it.
That's not to say that if you're not the World's Most Perfect Christian you don't count as a Christian. There's a difference between trying to live your life for Christ and screwing up (which we all do) and actually rejecting his teachings as you see fit. I'll never be a perfect Christian as long as I live. There's just no way. But rejecting his teachings to suit my lifestyle kind of takes me off the path altogether.
Like I said, you can believe whatever you want. You can believe an amalgamation of whatever religion du jour you prefer.
But unless you adhere to the teachings of Christ—all of them—you can't really call it Christianity.