Or, spelled correctly, “atheist defines Christianity,” although I don’t know if I’d have to. The basic notion of a Christian is one who’s accepted Christ as his or her personal saviour. The base and bottom line, that’s all you have to do to qualify. Christ died to absolve everyone of sin, so long as they promised to give him the credit for that magnanimous gesture.
At least, that’s what Christians say. I don’t know what order the books were written, if Paul’s collection of letters predates the gospels or the other way around. There are verses in the gospels that suggest it and Paul (or one writing under the name later) came up with other “proof” this was so.
Beyond that, every Christian sect will add in its own definitions of what makes it the right version to follow, be it Protestant, Catholic, or fundamentalist in some other way.
An atheist might choose to define Christians by what they do instead. Atheists can read the bible, too, and see what Jesus said his followers should do and point to all the ways Christians might fail to comply. Love your enemies, turn the other cheek, etc.
I’m out of time for now. Perhaps readers can add to this?