Wizards don't believe in gods in the same way that most people don't find it necessary to believe in, say, tables. They know they're there, they know they're there for a purpose, they'd probably agree that they have a place in a well-organised universe, but they wouldn't see the point of believing, of going around saying "O great table, without whom we are as naught." Anyway, either the gods are there whether you believe in them or not, or exist only as a function of the belief, so either way you might as well ignore the whole business and, as it were, eat off your knees.