As a first approximation, I define belief not as the object of believing (a dogma, a program, etc.) but as the subject's investment in a proposition, the act of saying it and considering it as true.
Source: The Practice of Everyday Life, ch. 13, 1974.
-- Michel de Certeau, ( 1925-1986), French educator; He identified communication as the irreducible element in the politics of modern societies in The Capture of Speech.