Although some might argue that this means I am not a Christian (in that I do not hold all the "common doctrines of Christianity"), I think C.S. Lewis (from Mere Christianity) has the definition of "Christian" right:
Far deeper objections may be felt—and have been expressed—against my use of the word Christian to mean one who accepts the common doctrines of Christianity. People ask: 'Who are you, to lay down who is, and who is not a Christian?' or 'May not many a man who cannot believe these doctrines be far more truly a Christian, far closer to the spirit of Christ, than some who do?' Now this objection is in one sense very right, very charitable, very spiritual, very sensitive. It has every available quality except that of being useful. We simply cannot, without disaster, use language as these objectors want us to use it.