I think there is an anime exploring that idea. The Church would wield much more power- and most likely that Church would be Catholic or Orthodox as it already possess sacraments (physical things transmitting spiritual power) which that could fight them. They also both have a fairly centralized organization that is Global. Many Protestant Churches are not like this and may be overshadowed or simply unable to coordinate any meaningful resistance. It will have well trained exorcists already capable of fighting other evils. Sacramentals are the physical elements which are used to dispense the sacraments. 752
You could even combine blessed holy water or the relics of a Saint with Weapons (a Sword, or Gun's bullets..?) Vocations to the Priesthood would increase dramatically and all of a sudden people would give the Church heed on moral stances on sin.
Deacons, men having some vows but not fully priests, would probably become almost warrior like because they can handle some of the responsibilities of a priest (other than administering sacraments).
Governments would probably start trying to meddle in Church affairs again because Bishops are once again influential, powerful leaders. They'll start trying to appoint their favorite candidates for Bishop- this could lead to increased corruption. There would be a struggle for autonomy for the Church. On the plus side for the Church, they can make demands of secular governments and society.
Areas where the Church is already powerful will be the most protected in the short term and may become bases of operations from which to launch re-conquests of Vampire held areas. (example: Italy may be used to reconquer Northern Europe with Poland's help..)
Superstitions and the growing new Paganism may collapse if they can't replicate any of the power that the Church has. It may validate the existence and power of the God of <PERSON>.
I'd look to how the Church grew in influence at the beginning of the Dark Ages as a model for the transition from now to your setting as one reason it was like that was that Bishops were the only Roman officials who didn't abandon their stations; people rallied around their leadership. 274