Religious Statistics

It's agreed there are largely three main sects of Christianty--Catholcism, Orthodox, and Proteststant, with a few harder to peg demoninations, which I'll get to in a bit.

The Orthodox Church will probably see gains, maybe even doubled, but virtue of Russia's vast size, though it would likely peter out as the steppes become reclaimed by Islam and more Eastern and folk religions.

In the world of Medieval America, well, White's vision of neo-Medieval Europe, the Roman Church has made something of a comeback. Its population in Europe is probably comparable to that of the old Europe. It may have even reclaimed a great deal of territory of Northern Europe, as the relative irreligiosity and lack of centralization would make it not unlike the conversion of Germanic Pagans in the old Middle Ages. The biggest question mark would perhaps be England, but keep in mind its brand of Protestantism is not quite same as other denominations, and the Anglican Church would theoretically be easer to reunite. Probably something like 45 million if it matches its original Middle Ages figures. But it could also plausibly double, if not more, in the New World. I've estimated the population of Medieval Latin America at somewhere in the territory of 65 million or so. I think Mexico, Central America, Columbia, Venezuela and Ecuador are almost certainly to remain mainline Catholic. Once we start getting into more remote areas we can see its influence peter out, but I would definitely put money on 100 million Catholics worldwide--a significant boost, although arguably a decline in a worldwide market share.

It's interesting to consider where Protestantism would be. It's quite possible to consider it being all but wiped out, perhaps an interesting coda to the birth of the modern world being very much tied to Martin Luther's proclamation. Its lack of centralization and ties to ethnic groups would make it very hard to survive. Evangelical orders outside the West would slowly dissolves as local indigenous beliefs soaked back in. It's most likely to endure in Africa, where Christianity is mostly Protestant in nature, and where the lack of centralization could allow it to endure, in a way, with the more tribal structure. (Though it's also possible any organized Christian denominations would follow the lead of the Ethiopian Church). Perhaps the closest thing to true Protestantism in the New World (Barring any new movements in South America, is certainly possible) would be the six million or so adherents of New Israelite, which can be considered a part of, or close relative of Fundamentalism. Overall, the addition of Protestantism would be quite large considering the population in the Middle Ages was nothing, it could possibly lose its status as a "big three" 

American Non-Denominationalism is not Protestant. One could even call it anti-Protestant, as it was engineered as a specific move to consolidate and centralize all Faiths, with it defaulting to Christianity more than anything. That said, it has an impressive forty million or so in the fold, largely based in a single geographic area, and one that strongly defines America culturally and politically. It would be the distant but clear second largest sect of Christianity (though skeptics might claim it, like Mormonism, is not actually Christian) and quite possibly the fifth biggest sect any religion overall in the new Medieval world. That it is dominant but confined to one corner of the world makes it perhaps quite a parallel to the popularity of American Football in modern times.

Judaism would be larger than it was in the old Middle Ages, but would still likely be very spread out and hard to form into a cohesive political unit.

As for Islam, it's very unlikely it would leave a footprint in the Americas, as most of the practitioners in the Industrial era were based in cities, and urban populations which be much smaller and less self-sustaining in the medieval world. That said, the expansion into southern Africa and Asia that began in the old Middle Ages would have solidified by now, and one can see the overall Muslim population having doubled, perhaps even tripled, and Sunni Muslim becoming the single largest denomination in the world. However, it's also possible that Oceania's distance would have a strong effect on their practice (For instance, making to Mecca, and existing on the other side of the Equator) and leadership, and much like Non-Denom, National Identity and absorption of local pluralities would make Indonesian Islam quite distinct from Sunni and Sh'ia