Is atheism the fastest growing "religion" in the history of the United States?

Question

First of all, yes, atheism isn't a religion. Taking from the Wikipedia link, atheism is defined as

Most inclusively, atheism is simply the absence of belief that any deities exist.

Penn Jillette claims (at 5:54)

I believe, if you counted atheism as a religion, it's the fastest growing religion in the history of the United States of America.

I am doubting this. The US seems like such a religious country still, I find it hard to believe that atheism is growing that fast. Especially since atheist are the most distrusted group in America, ahead of gays, Muslims, and blacks.

For the purpose of answerability:

1. only openly declared atheists or non-believers are counted (for example, in a census or poll);
2. only large and/or officially recognized religions are counted (say, that have at least 1 million adherents in the US);
3. confounding factors ("more acceptable to come out") are ignored;
4. self-description is what counts (if you call yourself a Christian in a poll, you are a Christian for our purposes).
5. either proportional or absolute terms are counted.

Points 1. and 4. may seem arbitrary but I think they are strongly correlated/proportional to "true" believers. I think we can make the question answerable by assuming this as an objective criteria.

Point 2. removes the "yesterday it was me, today it's me and 4 friends: 500% growth!" factor.

Regarding point 3, let's make an arbitrary assumption that an "out" atheist counts "more" than a "closet" atheist, because in a sense it's still an increase of numbers. I am not sure it's right to correct for these, and that Penn Jillette claims otherwise. If you find real numbers to quantify these people do so for bonus points.

Regarding point 5, Jillette doesn't say what he means about this and I think an answer can easily cover both.

Answer 1

Here are the US Census Bureau Statistical Abstract data on religion for the last three issues.

Between 2001 and 2008, for the numbers of people giving their religion as:

• "Atheist" rose from 902,000 to 1,621,000 (+70%)
• "Atheist, Humanist or Agnostic" rose from 1,942,000 to 3,696,000 (+90%)
• "Atheist, Humanist,Agnostic or No Religion" rose from 25,428,000 to 34,119,000 (+34%)

Other religions clearly grew faster than all of those in terms of pure numbers. Christianity added nearly fifteen million. In percentage terms both "Atheism" and "No religion" outgrew most mainstream relgions. However the number of people giving their religion as:

• "Evangelical Christian" rose from 1,088,000 to 2,154,000 (+98%)
• "Nondenomination Christian" rose from 2,489,000 to 8,032,000 (+223%)
• "Wiccan" rose from 134,000 to 342,000 (+155%)
• "Pagan" rose from 149,000 to 340,000 (+142%)
• "Spiritualist" rose from 115,000 to 426,000 (+267%)

It is likely that many of those describing themselves as "Evangelical Christian" or "Nondenominational Christian" previously described themselves as some other form of Christian, so you may not want to include those. However it is clear that the claim is not in fact correct.

While Atheism grew faster than the main religions in percentage terms, a number of religions grew faster still

EDIT:There is a sense in which the statement could be perhaps true. "No religion" as a percentage of the population grew from 7.5% to 13.3%, the highest percentage increase of any category. However it is far from clear that "no religion" implies a disbelief in all deities, and self-declared atheists are counted separately from this, so this figure would not satisfy criterion 1 of the question.