Christianity is such a large religion that there isn't an ethnic group attached to it.
That is a bit tricky.
Religion is definitely a choice if you're enough willing to make a choice, but most people just keep their parent's religion. People just believe what they were taught as children.
I'm atheist from atheist country so I don't necessarily get it, but it seems to be that way - see ratios of Muslim and Christians in Saudi Arabia versus Europe, for example.
Then there's more tricky cases where religion and ethnicity are heavily correlated or tied in even stronger way (e.g. Jews).
Is that not a choice? Choosing atheism is arguably easier.
Hard disagree here. I didn't choose to be born to parents of a fundamental baptist cult. I didn't choose to be subjected to physical abuse in the name of that religion, or to be sent to seminars ingraining deeply harmful ideas about self and the roles of men/women. I didn't choose to spend my entire adult life in therapy unwinding the bizarre ideas that had been hammered into me as a kid.
Yes, you can "choose" to leave your religion behind, but many cannot choose to avoid the indoctrination, abuse and twisted mental models that can come from the experience.
I left my faith behind around age 18. 20 years later I'm still actively working on getting rid of the rest of the baggage.
Just because someone says you are a Christian because you were baptized or whatever doesn't make it so. Religion is just as much a choice as an occupation.