Christianity and Islam, in roughly equal amounts (around 33% each). However, in some countries like Russia and China, it looks like Christianity is taking the lead. But do be careful to differentiate between Christian denominations when considering this. Those numbers within different denominations tell a very different story; they’re not all equal when considering Protestants alone for example.
According to Christianity’s own seminary school, stated by an Expert from Harvard University, “About two-thirds of all Christians now live in the Global South and 431 million Christians currently reside in China.” And yet another expert states:
“the number of Muslims (1.57 billion) and Hindus (900 million) far exceeds that of any other religion”. With such complexities as these defining what a successful global religion is today, it may be hard for people to agree with each other as to which type of religious affiliation deserves this top spot.