They literally have the source of all the Farley church's blessings on their island, who happens to be a dude who would HATE the church once they told him what's happened. They have such a massive upper hand in any negotiation with the church that it's laughable.
The problem is its not really a upper hand. They are a small group vs pretty much the entire world. It doesn't matter what they know if they have no one to tell, none willing to learn. It doesn't matter what they have if they can't keep hold of it, the church has more blessed, has shear numbers. The church controls information of the world, and has the military might to do what they want. And worse its in a isolated island. They can spin this however they want. Worst case the source of the blessings gets killed, they claim it was heretics who caused the problem. And use it to justify strengthening their rule.
Basically they have info and they have something the church wants, but they lack options or ways to use it. His power is not convenient. He sees to have little to no control of it.
The key here is how is he a trump card vs this witch, who and what is the witch. Because a person who causes people to be born with powers is not a great trump card in a short time. Ether the church didn't know what his power is anymore, just that its important. Or there is more to his power.
