During WW2, Pius XII had written a resignation letter to take effect if the Nazis/Fascists arrested him, and that the Curia were under orders to move to Portugal in that event.
Let's say in 1941 when Hitler seemed invincible and irrefutable proof of atrocities was streaming into the Vatican. Pius XII decides to martyr himself. He makes a lengthy denunciation of fascist ideology and requires every parish priest in Nazi-occupied Europe to read out the statement on Sunday. All Catholics in Nazi-ruled Europe is demanded to refuse orders which violate Catholic doctrine. He had already secretly evacuated the Curia and the Vatican files to Lisbon, leaving him virtually alone.
He waits for the blackshirts. They pick him up and detain him in the mountains.
Meanwhile, Hitler is incensed. He orders all Catholic bishops in the Reich to be executed, and makes a speech denouncing Christians as spiritually degenerate unworthy of existence. Many clergy, Catholic and Protestant, are killed and/or sent to the camps. All churches are closed and their clergy defrocked, and church activities are forced underground. But many Gestapo officers are themselves unwilling to partake in the persecution and suffer reprisals themselves. Many Wehrmacht soldiers are court marshalled, make passionate defenses, and are executed on Hitler's orders.
Allied radio propaganda then frame the war in religious terms (a fight for Christendom against godless fascism). Even Stalin follows suit, with his propaganda highlighting how Jesus was a proto-Marxist in a feudal society and therefore extremely far-sighted.
After the war, the church is hailed as the only institution which kept Europe alive during the dark period. The martyrdom of so many Christians is hailed, and the anniversary of Pius XII's speech is celebrated across Europe and its offshoots. The Cold War starts with both sides claiming their ideology more properly represents Christianity, with the other as a distortion. Priests (both surviving and new ones) are as revered as war veterans IOTL. Hence, everyone, even those who don't literally believe a piece of flour is the flesh of a zombie who was miraculously born after a fairy said so, is up early on Sundays.