I think the number one reason is that many christians feel the need to push their beliefs onto others. Often the person doing this has no idea at all about their religion, and either grew up on it or goes to church occasionally, or likes the idea of being considered a christian because of the rewards it offers.

I've met very few true christians, but the ones I do know are hesitant to even call themselves christians because they are completely different from the others.

Also, christianity is the most popular religion in many countries... It's often the easiest one to refer to when making a point about spiritual or religious issues, because people know what you are talking about. I, personally, use it to argue against and refer to sometimes because I grew up as a "christian" and I know the bible decently, better than I know any other religious documents and scriptures.

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