That is surely the point. For some-one to say they're a Jew, Christian, or Muslim if they cannot accept the basic profession of faith is just being silly. Another poster compared it to stating your favourite colour, but it's more like saying you accept evolution. Either you believe humans have evolved or you don't. Similarly, either you believe Jesus is the Son of God and rose from the dead or you don't.
This even applies to religions that lack dogma. To say you're a Pagan implies that you believe in gods and are not an atheist.
Of course, there are always people here who believe they are entitled to call themselves whatever they like. Similarly, there are people who believe they're Napoleon...
This all seems reasonable. Who gets to say what is the basic profession of faith for any given religion?