The census is not a vote. It is not a popularity contest for religions. It is not a referendum on the "official" religion. It does not change the law. Even if 100% mark e.g. "Calathumpian", it won't change the government. The census informs government and policy makers. To do this well it needs to be accurate. Irrational fear - also of Muslims - is a bad advisor.
See also: http://bit.ly/r766rr