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The most likely answer is Hindu or Buddhist because the Islamic empire may have been big but they also never took the time to convert the places they took over to their religion
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While not exactly answering the question regarding POPULATION, I've found this good animation created by Business Insider showing religion SPREAD through the years.
https://www.youtube.com/watch?v=AvFl6UBZLv4
If we assume (probably incorrectly) that the spread is proportional to population, then around 1500 with the conquest of the New World, Christianity would be most wide-spread religion.
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The Taoist, Confucianist and Traditional religions of China (grouped together) were the largest religions by a huge margin, with (guesstimate) 115-125 million followers. Hinduism would be the single largest religion with about (guesstimate) 70-75 million followers. Christianity isnβt far behind at about 65-70 million followers. All these are guesstimates and hypothesised by looking at the population of certain empires at The Wikipedia page on the same. A few honourable mentions are Buddhism, Shintoism and the multiple religions of the Aztec and Incan empires.
However, Islam was the most widely spread religion in the world stretching from Morocco to Malacca.