Go back further. Rome breaks apart during the crisis of the third century, with Palmyra and Gaul surviving as separate entities, Constantine doesn't convert, the Edit of Milan expy is less generous to Christians, enshrining tolerance but not giving back property, handicapping some of the growth. Palmyra eventually converts but in Rome they remain a minority community. Ethiopia and Armenia still become Christianized.
If there is an Islam in this TL, it will likely take hold in Egypt and Syria, relegating the Palmyrene Christians to large minority status.
Fast forward to the present, Christianity is a substantial minority religion in Italy, Greece, Anatolia, Egypt, and North Africa, with a few Christian majority nations in East Africa and the Caucuses.
Enough to be a major world religion on the level of Hinduism or Judaism, not a major world player though.