Okay, here's a thought. Probably somewhat implausible, though, and requires a pre-1900 POD.
- In one of her 19th century wars against the Turks, Russia is able to triumph. Constantinople is captured, and a ring of vassal states are set up around the Balkans and Black Sea. Greece is granted swathes of extra territory and becomes a Russian vassal. Russia gains a warm water port, and Constantinople is set up as a "free city".
- A Great War of a sort breaks out, in which Russia is on the winning side, and takes the opportunity to directly annex several of its Balkan vassals, as well as expanding its territory in Pontic Anatolia and the Caucasus. Orthodoxy comes to be used as the glue to bind this wanked Russian Empire together.
- Socialist revolutions break out and succeed in several Western European nations. Though the ensuing dictatorships don't last forever, they do heavily weaken any sort of remaining Christian influence on Western Europe. Elements of colonial administrations remain in place in some colonies, ruling Apartheid style states that sometimes begin to resemble an "Angrezi Raj".
- By 2012, Russia is the most powerful nation on Earth, and greedily eyes territories in Western Europe and central Asia. Russian missionary activity is proving to be hugely successful in China, and Orthodoxy is on the rise everywhere. Constantinople becomes a city of the Patriarch, himself a puppet of the Russian-Greek alliance. The Patriarchate increasingly becomes a mouthpiece for Russian propaganda about her destiny of global dominion.
- The rest of the world cowers in terror.