what might lead Kino and his brother to believe in the priest? what might lead them to be suspicious of this comment?

Comment? The priest telling Kino that he will take the pearl to the capital himself?

If so, the priest says, "each man must remain faithful to his post and must not go running about, else the castle is in danger from the assaults of Hell." The translation to his poor natives is to stay at the bottom of society. Don't go trying to better yourself or you will end up in Hell. The Priest is pretty transparent about what a racist crook he is. The priest likes to surround himself with fine things and wear expensive robes while his congregation wallows in abject poverty.