Some background info: The Bab, divine manifestation and forerunner of Baha'u'llah (the "John the Baptist of the Baha'i faith", if you will), tought in Persia 1844-1850, when he was executed. During these and the following years, some 20,000 to 30,000 of his followers were persecuted and murdered.
Bábism - Wikipedia, the free encyclopedia
Baha'u'llah revealed His divine manifestation in 1863, in accordance with the Bab's announcement, and was consequently banned, exiled and incarcerated with his followers by the Muslim authorities.
Persecution of Bahá'ís - Wikipedia, the free encyclopedia
Despite this massive persecution, the Baha'i faith has spread, increased its number of followers from a couple of hundred thousands to between 6 and 8 million during the 20th century. That means there are already half as many Baha'i as there are Jews. Also, the Baha'i faith is today the geographically second most widespread religion on this planet.