Here, John means that arguing about God ultimately can’t work—God is above human categories, so human reason can’t apply to God successfully. He also rejects the idea that it’s necessary to understand something in order to believe in it. John isn’t being anti-intellectual; he values searching, studying, and asking questions about one’s beliefs. But he’s essentially discouraging his son from mixing up reason and faith—these categories might complement each other, but they can’t take each other’s place.