The Buddha didn't praise those who merely believe others, he praised the person who knows within himself. Just as with that fruit, if we have tasted it already, we don't have to ask anyone else if it's sweet or sour. Our problems are over. Why are they over? Because we see according to the truth. One who has realized the Dhamma is like one who has realized the sweetness or sourness of the fruit. All doubts are ended right here.