Not sure why. To be a Christian is to be a follower of Jesus Christ, or it's supposed to be.

Jesus did not advocate religion. In fact he was a fierce critic of the religion, especially his own. He saw them as inherently hypocritical.

That's technically not what theism is, but it's what most people take it as. If you go by the strict definition of theism, I still don't see the relevance. Are you able to work alongside people who don't believe in God at your place of employment? Would you refuse to buy groceries at the only store in town just because the proprietor was an atheist?

What's the difference?