You're partly correct.
In truth, the Church was started by witches who were survivor of the witch society that lived in much larger area but lost a war and the survivors escaped to this place (their servants went on to become founders of the 4 kingdoms)
Their goal was to basically gathered witches and turned them into Space Marines in a sense, to make the 'useless' non-combat witches into something combat-capable.
A body capable of nullifying magic and trained in combat, with the soul of the sacrificed witches who'll spend the years in preparation for the upcoming war against the Demons that drove them out (yes, the same demons that rekt'd Cara) because if they lose the next war, humanity will go extinct.
Basically it's one of those "The End Justify The Means" plan.
However, over time the successors to the church forgot their original plan, even forgetting that you're supposed to put the souls of the sacrificed witches into the body.
So instead of Space Marines, all they got was a very very strong and obedient zombies.
And of course many of the new human in charge are corrupted/greedy so the situation turn into what it is.
On the 'plus' side, the current Pope actually is trying to get a witch to be his successor, who'll direct the church to its original goal: formation of the immortal army for the war.
Eventually, they lose to Roland, and so the plan was scrapped and the remainders of people who's focused on the upcoming war join Roland's side one way or another (the corrupted members ran off into hiding)