God is bound to do what he says, according to Jacob.
O the greatness and the justice of our God! For he executeth all his words, and they have gone forth out of his mouth, and his law must be fulfilled.
Jacob is saying that every word God speaks must be executed. His law must be fulfilled and the righteous will receive a righteous reward and the filthy will receive a filthy reward.
One can see the wisdom God had when He put forth that He was going to require a Saviour. He knew that we would all sin and fall short and that He would be obligated and bound to make sure our sins were reciprocated with judgement. Yet, just because He is just and all His words are fulfilled did not mean He could not show any mercy.