The way I understand it is this: It's the same as building a castle upgrade. It will contribute -1 to your overall food. If you have 0, it'll take you to -1 overall, and so on. Upgrading your farms is the best way to ensure you always have enough food.
Churches and certain buildings will provide a conversion rate to Christianity as well. It's also possible neighboring provinces are also contributing. Mouse over the religious icons in the upper left of the settlement tab to see the various conversion factors at work.I'm also having a problem still with Christianity. I've noticed that when I've taken over a province I will put a monk or two in it. Christianity will go on the decline, an as soon as I pull out the monks Christianity goes back up. I don't have one of those trading ports which will causes an increase in Christianity, so I'm stumped on that one.
Thanks
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