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According to Wikipedia, the only religions with over a hundred million followers globally are Christianity, Islam, Hinduism, Buddhism, and various folk religions (this is, in my understanding, an umbrella term that includes many different faiths), while borderline examples include Taoism, Shinto, and Falun Gong.

So, with a POD no earlier than, say, Constantine's issuing of the Edict of Milan in 313, what other religious traditions had the best chance of garnering a hundred million believers by 2018?

Here are a few of my thoughts, which by no means constitute an exhaustive list...

Judaism: I've read that lessening the level of persecution that the Jewish people suffered over the centuries would lead to a much higher Jewish population today, but there are other ways to augment these numbers as well. Several countries in the Classical and medieval eras either converted to Judaism, or had the potential to - the Khazars are perhaps the biggest example, but there was serious potential for this in several places in the Horn of Africa and the Arabian peninsula as well, and this isn't even mentioning longshot options such as Vladimir the Great toying with the idea and converting the Kievan Rus to Judaism rather than Christianity.

Zoroastrianism: Would this be as simple as preventing the Islamic conquest of Persia, perhaps coupled with strong Persian influence over the centuries in Central Asia, South Asia and the Indian Ocean?

Arian Christianity: Technically a form of Christianity, but one considered heretical by Chalcedonian Christians, so I'm throwing this in for consideration. It was very popular among many of the Germanic tribes that invaded the Roman Empire (Goths, Vandals, etc), but they didn't survive as a major force for more than a few centuries after the fall of the WRE. Maybe in a world where Justinian hadn't tried to reconquer the WRE, thus destroying the Arian Vandal and Ostrogothic Kingdoms and isolating the Arian Visigothic Kingdom, the rulers of those areas could've slowly converted much of the native population to Arianism, in the same way that Muslim rulers of the Middle East and North Africa only converted the Christian majority of those areas to Islam over the course of centuries?

Reformed European Paganism: I've seen this suggested as an option in other threads, both in the context of Germanic/Norse paganism and Slavic paganism. Though I'm skeptical about the viability of this, the gist of these scenarios involves a major religious leader creating more organized doctrines and heirarchies for these pagan peoples, thus allowing them to better resist Christian missionary efforts. The problem, at least with Norse paganism, is that even if this is technically possible, the Scandinavian countries have a small population that makes it difficult for this to ever achieve the status of major world religion, even if you spot them a pagan equivalent of Cnut's Empire of the Norse that converts Britain and perhaps some other peripheral areas of Europe, though making some successor state a major colonial empire globally could solve this issue.

Tengriism: Religion of many Central Asian peoples, including the ancestors of the Hungarians and Bulgarians. Like European paganism, you might need to spot them a major religious figure/organizer, but beyond that, I imagine that you'd also need to prevent the Islamic conquests of Central Asia. That, and/or a more chaotic religious situation in Europe taking the pressure off of the Magyars and Bulgars to convert to Christianity.

Sikhism: Perhaps in a world with much weaker British presence in India, the Sikh Empire manages to consolidate its rule in OTL Pakistan and slowly convert a majority of the population?

What are your thoughts on these example - plausible, or not? Additionally, what are other religions you think could've gotten over a hundred million followers?
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