I was nominally raised in the Episcopal Church, but never confirmed. There's an order of confirmation and a creed, though, which outline the basic belief requirements of being a good Episcopalian. This is also present in the Lutheran Church and originated with the Roman Catholic Church.

So for some groups it's very easy to determine what makes one a Christian -- Christianity consists in a certain set of beliefs outlined in the creed of the particular Church. The most common creed is probably [link|http://www.reformed.org/documents/index.html?mainframe=apostles_creed.html|the Apostles' Creed], but it is by no means universal.

In certain branches of Protestantism these days, one can generally get away with the vague "I accept Jesus as my savior", and heavy emphasis is then placed on personal interpretation of one's relationship with Jesus.