18 But when their heathenism from being inward became open, then Moses also from being hidden openly appeared; that he might openly punish those whose heathenism had revelled beneath the holy cloud which had overshadowed them. But God removed the Shepherd of the flock from it for forty days, that the flock might show that its trust was fixed upon the calf. While God was feeding the flock with all delights, it chose for itself as its Shepherd the calf, which was not able even to eat. Moses who kept them in awe was removed from them, that the idolatry might cry aloud in their mouths, which the restraint of Moses had kept down in their hearts. For they cried: Make us gods, to go before us.
Source: On Our Lord (New Advent)