Lest you corrupt yourselves and make a graven image

(Deut. 4:16)

Why did Moses have to remind the Jewish people not to make graven images, considering the fact that they had just spent forty years in the desert and had seen all sorts of open miracles and wonders? Were they not already on such a high spiritual level that making a graven image would be unthinkable? From this we learn that an individual must never think that his worship of G-d is perfect and he is beyond temptation. One must be ever on guard, even against those sins which appear to have no attraction whatsoever. (Sifrei Musar)