The Baha'i Faith was the fastest growing religion in the twentieth century.
Certainly this seems very important to Baha'is as it's brought up a lot. I personally don't believe it, yet understand why struggling groups would want to make it seem that way, especially for their own followers to retain confidence in the movement. But it has no basis in fact at all, is another perpetrated myth. Of course if you're Baha'i, you have to believe it, because your leaders say it is so. But non-Baha'i are free to look at facts
Here are some other examples. Note that most of them don't put emphasis on declaring themselves a new religion, emphasis is on deeds usually. Less emphasis generally on the leader too, and far less emphasis on spreading their personal faith, or way.
Swaminarayan BAPS ... Many mainstream Hindus consider them as non-Hindu, a radical offshoot. (Most of the Indian groups I will mention parallel Baha'is relationship to Islam, roots in Hinduism, but openly declare themselves non-Hindu) Some sources say 88 million followers, (that's very debatable, but it's higher than Baha'i almost for sure) started around the turn of the last century, amazing charity work, lots of awards and recognition, hold 33 billion in assets.
Satya Sai organisation - in 130 countries, lots of charity, 1200 centers , claim over 10 million followers, decidedly universalist, started about 50 years ago.
Brahma Kumaris - in 110 countries, UN consulting status, 3 million + membership, openly non-Hindu
ISHA Foundation - 5 million volunteers, started 1992, universalist, tons of charity work, founded in 1992
Amritavarshan - founded 40 years ago, in 191 countries, tons of charity, and recognition by UN, and others. Universalist
Amaddiya Muslim ... like Baha'i, considered non Muslim by much of mainstream Islam, 15 million followers world wide.
Besides all this, the Baha'i faith may not even be growing at all. It's really hard to tell. Some non-Baha'i' or ex-Baha'i claim it's shrinking, Baha'i' say it's slowly growing. Obviously they would say that, it's an agenda. Non-Baha'i have no agenda. Ex-Baha'i may have an agenda. (By agenda, I mean a reason to distort things)
None of the organisations I mentioned, as far as I know, go around boasting about their growth, although some individuals surely would.
Not mentioning religions, some individuals have had a huge impact on the world than entire religions. Deepak Chopra in giving basic mental training comes to mind, and certainly individual plhilanthropy like Bill Gates do more charity.
So decide for yourself.