The OP asked for the shortest answer possible, so that's mine. I don't ask people what they believe right after I meet them (I've met plenty of people who do...) but if they act as Jesus would have, that's more Christian than talking about him but not acting.
So it's not my definition of Christianity, but I think it's the most important requirement. The Lord might disagree, but He said my reason was right up there.
Erik's post, linked to earlier in the thread, gives an in-depth look at many aspects which I think are important to living as Christian a life as possible, but I wasn't just going to write "what Erik said PC +1," was I?