10 And not only this, but when Rebecca also had conceived by one man, even by our father Isaac
Paul moved from the first illustration of Isaac and Ishmael (vv. 7-9) to answer an objection that came out of the previous illustration. Some might object that Ishmael was not a true Jew. His mother was Hagar the slave, not the son of Sarah. Some claimed that that is the reason God did not choose him.
This second illustration in verses 10 to 13 demonstrates that, because Esau and Jacob were twins and had the same father and mother, the choice of Jacob was God’s prerogative. Ismael did not have the same mother as Isaac, but Esau had the same mother as Jacob. This passage says in effect that physical legacy was not the basis of God’s choice. The choice rested on His plan revolving around his covenant.
10 And not only this,
With further particularity in another illustration, Paul continued the same thought of the way of God’s salvation. “Consider the case of Jacob and Esau in addition to the case of Isaac and Ishmael.” Paul noted differences in Abraham’s posterity in verses seven to nine, but now he showed that there was no difference in Isaac’s posterity because Jacob and Esau had the same mother and father. Nothing changes God’s plan of salvation. God is always true to Himself. This is true theodicy.
but when Rebecca also had conceived by one man, even by our father Isaac
This phrase refers to Genesis 25:21, where God answered Isaac’s prayer to have a child.
Rebecca had sex with one man, Isaac, and by one sexual act conceived both Esau and Jacob. There is no question here of either child’s legitimacy. There was no human reason why God preferred one child over another. They had the same mother and father.
The solid ground of why God chooses whom He chooses is faithfulness to His promise (Ro 11:26-27). He is always true to Himself.
Ro 11:26-27, 26 And so all Israel will be saved, as it is written: “The Deliverer will come out of Zion, And He will turn away ungodliness from Jacob; 27 For this is My covenant with them, When I take away their sins.”
Religion has no part in salvation, which comes only through the grace of God.
God’s plan of election always revolves around the grace principle:
Ro 11:5-6, 5 Even so then, at this present time there is a remnant according to the election of grace. 6 And if by grace, then it is no longer of works; otherwise grace is no longer grace. But if it is of works, it is no longer grace; otherwise work is no longer work.