So they hired a potential spy raised by their enemies to be the poison tester for the saintess that keeps the king healthy? How dumb are they? All the church would need to do is raise a child while slowly feeding them some particular type of poison (the kind you can build resistance against), replace the poison tester that was going to be sent with the trained child, and then sprinkle the saintess' food with that poison. The poison tester will eat it but not show it, because they're used to it, and then the saintess will get poisoned. Without the saintess the king will weaken, and then the church can start a crusade or whatever.
The author made the setting so convoluted that the main character's reason for being there in the first place makes no sense.