It would displace all of the competition from other religions in the Middle East, and from thence into Central Asia and all of Africa. Basically only Asia screened by India and China would remain non-Christian, with the except of places like the East Indies, large swathes of India, and possibly Korea and Japan. With exceptionally good luck, however, I could see Christianity also becoming dominant in India and China and from there it'll become globally dominant.
What about Persia and its Zoroastrianism?
IIRC, most of the evidence puts to it being sharply on the decline, which was a factor in the Islamic conquest being so successful; Mesopotamia, which was the heart of the Empire, was probably majority Christian by the time the Arabs arrived.
I think that if Islam didn't happen, that perhaps some other religion would've popped up and would have had a similar effect in it's place.
Of course, if Persia goes Christian, that'll be Nestorianism. That's what was thriving in Mesopotamia historically.
This suggests that we see an enormous, possibly globe-spanning Christendom, but divided between a Chalcedonian sphere and a Nestorian sphere. (Both may well see internal schisms later on, of course.)
I would expect neither Orthodox or Eastern Christianitiy to reach SEA, but its Oriental guise.Indonesia, Malaysia, and other parts of Oceania and SE Asia
And certain aspects of almost every pagan cult ever - Spring Equinox and Winter Solstice - have survived in Christianity to this day ...some aspects of Zoroastrianism, such as Nowruz, have survived in Iran up to the present-day, even after over 1,350 years of Islamic rule there.
It would displace all of the competition from other religions in the Middle East, and from thence into Central Asia and all of Africa. Basically only Asia screened by India and China would remain non-Christian, with the exception of places like the East Indies, large swathes of India, and possibly Korea and Japan. With exceptionally good luck, however, I could see Christianity also becoming dominant in India and China and from there it'll become globally dominant.
No Jesuits here. Roman Bishop remains one of the Five Patriarchs.
Go Orthodoxy!