William Carey, missionary in India, and also William Wilberforce, were campaigners against the terrible Hindu funeral practice 'Sati' - the burning of widows on the funeral pyre of their late husband - which was common in India at the time (the late 18th century).
"Distressed at the excepting of Sati, Carey sent that year careful investigators to every village within 30 miles of Calcutta, to learn how many widows had been immolated there in the previous 12 months, and their ages (they were sometimes mere girls) and the children they had left doubly-orphaned behind them.
Four hundred and thirty-eight was the damning total in this specific area alone, the toll of a single year's superstition, cruelty, and waste...
The Mission never ceased to lodge its protests, not to publish the detailed death-rolls; but not for another quarter of a century were the Hindu widows redeemed."
(Source: William Carey by S Pearce Carey, 1923, p 209-210)
Dr Kate Bushnell
In the 1880s, a Christian lady, Dr Kate Bushnell, became aware of the massive issue of forced prostitution in the USA. She tried to raise the issue with the authorities, but as they supported much of the business, she resorted to investigating the conditions herself; entering brothels and interviewing the women.
Dr Bushnell reported her findings, which the state denied, but she persisted and the country became 'agitated on the white slave question'. Dr Bushnell's efforts started the process of banning forced prostitution in the USA, making it a better country for those who live in it today.