"Surely the thing that defines a religion is sincere belief, nothing else.
If you sincerely believe in something/anything then who are "they" to say it is or isn't legitimately a religion?"
And there in lies the problem - what if your sincere "belief" is that some element of society is wrong and that it's your duty to execute them. Does that grant you the freedom to follow your religion?
There needs to be a consensus of acceptable behaviour, and Pastafarianism is a political tool to show a mirror to religion and religious beliefs. If there is anyone on the planet who truly believes in the FSM then they've missed the point completely.
Biting the hand that feeds IT
