Grace reigns supreme over law, sin, and death. Because God is gracious and the supreme sovereign over His creation, and because He is supreme over law as its Giver and can resurrect whom He chooses, grace is His to give freely as He pleases. Grace is supreme over the others because God has willed it so and gives it to whomsoever He chooses.
Because grace is a gift, it can neither be demanded nor earned (it can, however, be requested). Therefore salvation must be by grace. Because of this, even the greatest sinner is not beyond the reach of His mercy. Conversely, because salvation is by grace, all boasting is likewise excluded.
For example, Isaac receives grace, but Ishmael is cast out with his mother. Jacob receives the inheritance and blessing, but Esau is in reality cursed. God chooses to have Christ born in the tiny town of Bethlehem, not at the Temple or even in the capital city, Jerusalem. He could have sent angels to announce His Son's birth in every capital of every nation on earth, or at least to announce it to the religious leaders among the Jews. Instead, He chooses to invite common shepherds and foreign magi for that peculiar honor.
John W. Ritenbaugh
The Sovereignty of God: Part Three