Arabeus Malva's face is shocked, sad, desperate... What an actor he is. He doesn't really care about what he's going to do. More than that, I guess he's glad he found a way to get rid of a competitor.
«The brightest star can hide darkness,» he says. «Just like, sometimes, the best people can commit mistakes. Our High Priest, the one guiding us towards the light, has fallen under the devils' coaxing. The Saint Adelphium himself has fallen under the wiles of darkness and tried freeing these lost souls.»
I can picture him sneaking in to free the kids. Even though it all might be a lie, just an excuse to punish him.
By the way, right now, I guess I can trust him. He wasn't manipulating me into believing him. He was honest.
So, it's true not everyone in the Church is evil. And there is a faction unhappy about how they treat Mages, even those in their ranks. Everything he said is true; this should be enough proof to trust him.