Prior to the colonization of the Americas and Africa by the West, it appears somewhat unlikely that Christianity was the largest religion in the world. If I am not mistaken, the majority of the world's Christians (over 90%) lay at this time in Europe.
According to Wikipedia, South Asia and East Asia had much larger populations than Europe. Since South Asia was mostly either Hindu or Buddhist (with Muslims being a very small minority) and East Asia a combination of Confucian/Buddhist/Taoist..etc, does it not follow that one of these was the major religion? If we add Islam into the equation, it is also apparent that Muslims were perhaps the most geographically widespread community on the globe at the time, stretching from Morocco to Kazakhstan to the Malay Archipelago. However, it does not necessarily follow that they were the most populous.
Either way, it appears to me highly unlikely that Christianity was the largest religion in the world at this time. Even a cursory look at the statistics makes this situation improbable.
Christianity according to ChristianityView became the largest religion in the world only in 1900. This is for me more rational, due to the Russian intrusions into deep Siberia in the 16th century and further Christianization of these areas and of course the North and South Americas along with many large chucks of Africa being converted to Christianity. In 1800, the page suggests that Chinese folk-religionists were the largest religious group. I don't know exactly with what evidence this has been suggested but was this true for 1500 also? Can someone verify the data provided by this page?
With all matter aside, it appears that nobody has actually considered this problem more rigorously. It would be extremely interesting if it was to be dealt with.