Several characters' bodies slowly turn into a crystallije substance, a process known as "corruption". One character dies from this, and the character's body soldifies completely.
Not onscreen, but one character provides sidequests that involve avenging various villagers that have died in the invasion. The list starts with her husband.
For most of the game religion is mostly just a background item: the enemies are demons from Hell, some bosses and enemies are named after demons from the Ars Goetia, and the Church is involved in fighting them, and that's as far as it goes. But this changes as you approach the game's last enemies, as a major character loses faith and begins to plot deicide.