Indonesia, Malaysia, and other parts of Oceania and SE Asia
I would expect neither Orthodox or Eastern Christianitiy to reach SEA, but its Oriental guise.
In OTL this area went Hinduistic and/or Buddhist. In spite of having access to Christianity for over half a thousand years. And Islam initially spread through missionary work, not the sword.
Mesopotamia was heavily Christianised as, AFAIK, the locals were drifting into Nestorianism from various pagan faiths. Not switching from Zoroastrism. I'd expect Greater Iran to remain Zoroastrian (west) and Buddhist (east).
In India it seems that the Buddhists (although this is a complicated story) drifted into Islam or back into Hinduism. Maybe here more find Jesus?
With no Islam it is highly likely that Christianity would be more popular in the Great Steppe. Orthodoxy in the west and Nestorianism in the east. To what extent we cannot say. Buddhism, Tengrism, Manicheism all are competitors.
So, IMO "no Islam" = a Christian Africa and a more-than-OTL Christian Central Asia.
There seems to a broad consensus on this
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.
And certain aspects of almost every pagan cult ever - Spring Equinox and Winter Solstice - have survived in Christianity to this day ...