Originally Posted by Mongo View Post
Sikhism is already fairly close, with ~30 million adherents. It was founded in 1699 as a reaction to Islam, incorporating a number of Abrahamic tenets but remaining a Dharmic religion. It definitely has a reputation as being a "warrior religion", the Sikhs are not noted for being pacifists.
If it were slightly more popular, and its adherents had succeeded in fully holding back the tide of Islam, it could certainly have spread through much of Southern and Central Asia, meeting the requirements of the OP.
Originally Posted by Ganesha View Post
Sikhism does sound like a good candidate.
There's a problem, however, which it shares with the Baha'i faith.
Neither religion proselytizes. One of the main reasons Christianity and Islam were so phenomenally successful, and continue to be, is that they actively seek out new converts. Without that, growth is a much slower process.
.....
Cheers,
Ganesha
The thing is, while Sikhism isn't nearly as active and aggressive in proselytizing as Christianity or Islam (or Mormonism) are- as one would should really expect from a religion which cites freedom of faith as a basic human right- it still does proselytize. Further back in its history, it was a far more evangelical faith than it is now- especially during the Sikh Empire's rule, when it's estimated that the Sikh share of the religious demographic in their empire- which extended across the Indian and Pakistani regions of Punjab, Kashmir, and Pakhtunkwa, coming up to a estimated population of 23>24M, or roughly 1.9% of the total world population- increased almost sixfold, from around 3% to 17% in the space of only 4 decades.
Since the conquest of its empire by the British East India Company, Sikhism's share of the world population has remained almost constant at roughly 0.35%; but if the Sikh Empire had endured longer, Sikhism would certainly have been a larger religion, perhaps even coming to hold a majority within their own kingdom if given enough time. If we look at the present-day geographic footprint of the Sikh Empire, it's currently home to almost 170M people (i.r.o. 2.4%W) IOTL. In a timeline where the Sikh Empire survived to the present day, the 100M mark might even be attainable without going out of the way to seek new converts further afield.
Even if we rule out the slightly (but not entirely) ASB possibility of such a Sikh Empire expanding its territories further over the years though, a Sikh missionary ITTL wouldn't necessarily be limited to focusing their efforts within their own borders. India, particularly Rajasthan, Himachal Pradesh and the rest of IOTL's Pakistan- adherence to Sikhism and public displays of faith may well become restricted, even persecuted to a degree the longer that the British Raj remains in charge and Sikh Raj remains free, with the religion becoming associated with resistance and anti-imperialism; but this association would also be one of the greatest enticements to take up the faith among the wider Indian populace.
Afghanistan would have been a tougher nut to crack, but the independent kingdoms along their mutual borders- including the strategically located Wakhan, which the Afghans only brought under their rule in 1883 after the British told them to, and still encompassed almost half the area of Tajikistan to boot- could have offered fertile preaching grounds, serving as a staging point to spread the message along the trading routes to the vast plains of Central Asia and Xinjiang (/Yaqub Beg's 'Kashgaria', depending on when they got there) which lay beyond.
Final thought though; the reason why the world's 4 largest religions, Islam, Christianity, Hinduism and Buddhism, are all so big, isn't just because of how hard they preached. It's because of the regimes which supported their expansion, and which still do support them to the present day. Historically, the overwhelming majority of 1st generation members of these big religions weren't spiritually won over by preachers, they were convinced to convert by far more logical, practical arguments. Convinced by militias and lawmen, in order to avoid execution or incarceration; convinced by landowners, so that they could have places to live and grow food; convinced by bureaucrats, who imposed discriminatory laws and taxes upon those who failed to follow the official religion. In the Sikh Kingdom, where none of these professions reputedly displayed any religious bias, would mere societal pressure and national pride have been enough...?