originally, the church was estalished by witches as a desperate measure to unite humanity against a dooms day event.
Because they were betrayed by cowardly humans in the 1st war, they can't trust them.
Because they ran out of witches capable of fighting in 2nd war, they lost.
So after discovering technology from an ancient race, they found hope, two of them.
One plan was to turn as many 'useless' human and 'non combat' witches into anti-magic immortal warriors as they can (the human provide the body, the witch provide their blood + soul)
Though there's another group who want to find THE ONE witch who can activate a machine that in theory will wipe out the demons within massive AoE, thus winning them the war.
The first group became the one to established the church.
However, later generations of pope betrayed the witches and hid the truth, leaving only the pope in charge to know of the truth to continue the deception.
So right now much of the church aren't even aware of the demons, though some in the higher ups do.