Q98. "The Constitution proscribes an 'enumeration' every ten years in a manner proscribed by law, but no more. Aside from telling how many persons in my household, the rest of the questions appear 'unconstitutional' and I shouldn't be required to answer them."
A. Article 1, Section 2, Clause 3 requires that the government conduct a census every ten years. This is an actual count of all the persons in the United States.
The census was fixed at April 1, 1980, and every ten years thereafter. This decennial census is to be a head-count. A mid-decennial census, taken in 1985 and every ten years thereafter, is to be taken, and sampling methods may be used. Only the decennial census is to be used in the apportionment of Representatives. (13 USC 141)
The questions have nothing directly to do with the Constitution. The government has the authority to ask the questions in line with its duty to conduct the census, as well as in line with its other powers. You are required, by law, to submit to the census (13 USC 221), and can be fined up to $100 for not answering any census question. Giving a willfully false answer is punishable up to $500. The law does not require you to disclose information about your religion.