Bahai

Attempts to estimate the number of Bahá'ís face problems common to many religions. Unlike most other religions, the religion's governing bodies register new members and add them to voting lists. This is essential because the governing bodies are elected by all adult believers. But no religion has a way to track people who drift away without officially resigning their membership. In a few nations and regions (such as Bolivia, where there is a popular Bahá'í radio station, and Northern Ireland), the government census actually gives a membership number larger than the official Bahá'í statistic. In countries where the Bahá'í Faith is illegal (as in some Muslim countries) or where national infrastructure is very limited, it is difficult to obtain a reliable count of the Bahá'ís. Recent estimates of the worldwide Bahá'í population range from more than a million to upwards of seven million.The official Bahá'í number is five to six million.

Observers often notice a discrepancy between government and Bahá'í figures. Official Bahá'í statistics give a Bahá'í population in India of 2.2 million, whereas the 1991 Indian census recorded a mere 5,575 Bahá'ís living there. The World Christian Encyclopedia identifies the Bahá'í Faith as the second-most widespread religion in the world (Christianity being number one and Islam, number three).Because nine adult Bahá'ís will form a local "spiritual assembly" but nine adult Christians or Muslims will usually not establish a church or mosque, Bahá'ís are often more visible than their numbers would suggest.

It has been said that the Bahá'í religion is the fastest-growing world religion, based on the rapid growth the religion experienced in the late sixties and seventies.Mormons make a similar claim. Both claims refers to percentile growth rather than an absolute number of new believers; denominations and small religions often find it easier to achieve rapid percentile growth than larger ones. The Bahá'í Faith, historically, grows in spurts because of internal developments and external cultural factors. Its growth slowed in most areas of the world in the 1980s and 1990s, but has accelerated again since 2006. Overall, in the twentieth century its membership grew roughly fifty fold, from about 100,000 in 1900 to about five million in 2000.