The second largest religion by state in the United States (2012):
WebLink: BOINGBOING: Christianity is the top Religion
However, Buddha Dhamma is not a religion, in the sense of providing a set of guidelines to live by. It is about the natural laws of nature. Any person of any religious faith (or no faith in any religion, i.e., atheist) can follow Dhamma and benefit. All religions are about leading a good moral life, but Buddha Dhamma describes a much more comprehensive “world view” where this life of 100 years is just a blip.
If one becomes convinced of the truth of the Buddha’s world view, i.e., that this life is not the only we had or we will have, and that there are consequences to one’s actions, and the only way to get relief from inevitable suffering at least at the old age and death is to purify one’s mind, then the person is a Buddhist. It is all in one’s mind. No one else is tracking one’s progress, and no one else can purify one’s own mind.