I tried to give the Pope provinces time and again, but regardless of my standing with him, he always replied "we can't take this gift, it's too much!". Granted, most of the time I wanted to get rid of Oslo (playing the English, Oslo being a ceasefire gift from the late Scots), which was constantly being attacked by the Danes. But I even tried to get a buffer between me and Denmark by gifting Antwerp to the Pope, and he declined. Does the AI take into account the probability it can hold a piece of land you gift it with long-term?