Christianity is trapped between secular (but not necessarily irreligious) Classical Liberals and Social Democrats and Atheistic Communists and Fascists for most of the OTL. That’s the definition of being between a rock and a hard place.
Let’s have Russia avoid the Russo-Japanese War, have Franz Ferdinand take over in AH and get the League of Emperors going. Say the Royal families of Germany, AH, and Russia retain their level of religiosity, help keep each other in power, possibly staging coups to dismantle the power of their legislatures at some point, and they keep Christianity institutionalized. At this point, have Christianity retain its status as a part of people’s national and family identities. External enemies and competitors, like a secular America/Britain/France who constantly nag at everyone with NGOs and activists, and a resurgent Ottoman Empire who sponsors Islam, could help.
Hell, a surviving Tsarist Russia alone is half of Europe’s land and might make a majority of its population by 2019. A surviving Tsarist Russia might accomplish this alone if it remains as religious as the American South.
If the Balkan and Portuguese Monarchies survive, or a Franco emerges in Spain, and the Italian monarchy survives and improves relations with the Vatican they might fit into the above as well. But Russia/Germany/Austria-Hungary working together and surviving with Christianity institutionalized would be enough to accomplish this demographically.
But the overwhelming political momentum is against Christianity by this point. The Fascist movements are downright hostile to Christianity to an even greater extent than Liberals. But I’ve always wondered about an alt Hitler’s with other ideologies (Classical Liberal Hitler who conquers Europe to legalize freedom of speech and establish a right to bare arms everywhere, Communist Hitler who invades the Soviet Union for betraying real Communism, Social Democratic Hitler who invades Europe and brings legislature, feminism, and welfare programs everywhere as his goal), if an alt Hitler became a devout Christian, forgot about living space, and invaded Europe with a pan Christian movement aimed at destroying Communism and secularism across Europe...It would be interesting. Probably discrediting, but if he actually won who knows.