Let's keep the point of divergence simple: imagine a world where Muhammad had simply never been born. In such a scenario, what religion is most likely to be the most significant competitor to Christianity in terms of number of followers by the year 2021? Said religion does not have to garner around as many adherents as Islam does in our world - merely achieving a much more distant second place to Christianity would suffice. Options include...
- Zoroastrianism: The state religion of the powerful Persian Empire before the Islamic conquest of that country.
- Buddhism: Was already spread widely throughout Eurasia well before the point of divergence, and indeed, could plausibly spread in areas of Muslim dominance in our world (Central Asia, the islands comprising OTL Indonesia, etc).
- Manichaeism: A dualist, proselytizing faith that was also spread widely throughout Eurasia before the rise of Islam.
- Hinduism: Now, certain scholars might quibble as to what point in history exactly we can truly speak of this as a distinct, coherent religion, but you take the point - we are talking about the Vedic faith indigenous to South Asia, and which was present in other parts of the world such as Southeast Asia before the arrival of Islam.
- Surviving/reformed/organized European paganism: Though Christianity was definitely on the upswing in Europe by the time that Islam really arrived on the scene, I do not think that some significant survival of Germanic, Slavic, and/or Baltic paganism was impossible by that point. It is certainly possible to imagine a scenario where, say, the Vikings introduce some brand of their faith to Britain and/or continental Europe that at least slows the spread of Christianity, and could even spread further by way of colonialism.
- Tengrism: The native religion of the Turkic peoples before they largely converted to Islam, and which they may very well had stuck with in its absence. A large Turkic state in Eurasia, perhaps as an allohistorical counterpart to Russia, or even a Tengrist equivalent of the Ottoman Empire, could help spread the faith further.
- Judaism: Hard to accomplish, but worth mentioning as a possibility if only because at least the leadership of certain kingdoms, from the Khazars to the Himyarites, had converted at some point.
- Other religion that exists in our world: Say, Chinese folk religion or Aztec paganism.
- Other religion that does NOT exist in our world: Perhaps an Abrahamic religion that did not arise in our timeline that forms at the periphery of the Christian world.