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