Thank God the US Doesn't Count Religions
As Stephen Richter points out in passing nobody really knows what percentage of New York's ninth Congressional district is Jewish. And that's a good thing.
The United States government--which tracks things like the number of farms of 100 to 499 acres in Missouri--does not keep any statistics on people's religious persuasions. In fact, the keeping of such records is expressly forbidden by federal statute.
It's easy to see why: in a state that separates itself from the church, asking people about their religions could cause all kinds of difficult problems.
How would the government decide (or estimate) people's religions when they decline to answer? Guess work? How would a survey deal with people who were born into a religion but don't belong to a congregation and may not even believe whatever that faith preaches? What about people--mostly adherents of Asian faiths--who legitimately could and do consider themselves full members of more than one religion? Do you double count them? Not count them at all? Whatever decision got made, the government would be taking sides on a fundamentally spiritual issue.
And it could get worse. Think, for example, about how religious groups would get angry with the government when official statistics showed their numbers declining. They would have all sorts of reasons to air their grievances and claim (maybe with reason, in some cases) that the government had sided with whichever faiths were said to be growing.
What about internal schisms within faith traditions? Should members of polygamist cults that read the Book of Mormon be counted in the same category as people who belong to the mainstream Church of Jesus Christ of Latter Day Saints? Should "Jews for Jesus" be considered Jewish (as they consider themselves) or Christian (as almost all Jews consider them)? How would counting Catholics as the Christians they consider themselves to be anger the evangelical groups that consider their faith the only true Christianity?
In theological seminaries and religious school classes, all of these things are valid topics for debate and discussion. But they're also just the sorts of questions that have caused religious wars throughout human history. Thank God the United States doesn't even ask about religion.