Why do the heathen rage?
They rage because they believe the lie of the serpent, that they are gods already, and therefore need no salvation from sin. The audacity of a God who would become incarnate in the most humble of ways in order to enact this salvation infuriates them.
God Incarnate also takes away one of their biggest excuses for disobeying God - that he is a cold, distant, and indifferent deity who doesn’t know what it’s like to be human.