Why does God keep going back to the patriarchs when He deals with their descendants?
God repeatedly returns to the patriarchs because His love for them and His calling remain unchanged. Paul teaches that the remnant of Israel is beloved for the sake of Abraham, Isaac, Jacob, Moses, and the others. The gifts and calling of God are without repentance, meaning God does not change His mind concerning Israel.
Les explains that God has to constantly go back to those patriarchs and His love for them when He deals with their offspring. Paul is saying that the election, that remnant of Israel, which God is still preparing for end-time events, is beloved because of their forefathers—Abraham, Isaac, Jacob, Moses, and so on. Les then connects this to verse 29, where Paul writes that the gifts and calling of God are without repentance. Les clarifies that repentance in this context is not falling down on your face and weeping in sorrow for sin; it is a change of mind, a change of thinking. So when it comes to God's dealing with Israel, He has not changed His mind. The verse means the gifts and calling of God are without God changing His mind. Les points back to the Old Testament, Genesis 12, as what it is all referring back to.
As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers’ sakes.