Well to presume means to take for granted as fact.
To presume on God's goodness would be to expect him to be nice and good in all situations and taking for granted that his good.
Example of presuming God's goodness would be like saying: "I can sin as much as I want and will always be forgiven because he will be good and forgive me." or saying "I wont have to suffer like them because god is good and never lets anyone suffer."
