Why are people so bent on defending Obama's right to say he is a christian and not a Muslim, when religion....?

...religion period is not supposed to be an issue at all? If a person thinks he is a Muslim, what does it matter if there is suppose to be separation of church and state? So Obama is a Christian, so what why is that relevant and imperative to the potus? Why is it a plus for Obama to say he is Christian, but a negative for other politicians?