[Kluny]

You're not required to disclose your religion on the census. You can leave the field blank if you get the long form, and it isn't asked on the short form.

[slededit]

You are required to according to the law. The fact that it's rarely enforced does not change this.

Further if fields really were optional then it's not really "mandatory".

[wvenable]

The form no longer contains a question on religion.

[ultramancool]

It doesn't this year, but they say it will every 10 years.