Romans 9:15

Then he proceeded to refute the charge. When the whole nation of Israel rebelled against God by worshipping the golden calf (Exodus 32), God took the lives of only 3,000 of the rebels. He could have justly slain the whole nation. His mercy caused Him to do something that appeared to be unjust. Likewise in His dealings with Jacob and Esau God blessed Esau greatly as a descendant of Abraham, as He did all of Abraham’s descendants. Nevertheless He chose to bestow special grace on Jacob.

"The grace of God has been spoken of in this Epistle often before; but not until these chapters is mercy named; and until mercy is understood, grace cannot be fully appreciated." [Note: Newell, p. 355.]