Going from Romans 8 to Romans 9 is kind of like going from your favorite weather to your least favorite weather. It’s like going from your birthday to April 15 (unless April 15 is your birthday). It’s like going from a world of chocolate—to no chocolate, (Now I have your attention!).

I think our main challenge with Romans 9-11 is simple—we’re not able to connect with it as easily as we do with Romans 8. Although Paul is talking about something that’s quite relevant today (the status of Israel), on the surface we don’t see it as having much (or anything) to do with our day-to-day lives. But as we’ll see, this section of Scripture has some very important and basic truths for us.
The central issue of the section is once again, God’s righteousness. It revolves around the sad truth that although God had originally given the promises to and through Israel (9:4)—the bulk of the nation was in Paul’s time, unblessed (outside the Messiah). If that was the case, how could God possibly have been faithful to His people? How could He be righteous?
This is not just a first century issue because the answer for many people today is that if Israel is unblessed, it’s only because God has great plans for them in the future. At least part of the reason the nation has a generally positive political status is due to the belief of some that God still has special plans for the nation. And of course, there’s a huge chunk of contemporary theology that is dedicated to the idea that God isn’t finished with Israel. So, these chapters are certainly relevant to our world today and that makes it important to be biblically informed on the subject.
Moving on—what is Paul’s response to the charge that God has somehow been unfaithful? His explanation is simple and straightforward—God has been faithful to His promises (v. 6). The real problem is that the promises have been twisted and distorted by Israel to mean something that has never been true—namely, that physical descent from Abraham was all that was needed to be pleasing to God and inherit the promises.
Paul will break this falsehood apart piece by piece in a wide-ranging discussion of biblical election in v. 6-33. Here are the main points of his response (adapted from Mark Copeland).

1. The children of God are children of promise, not children of flesh (v. 7-13). He ushers in Abraham, Ishmael, and Isaac as Exhibit A in v. 7-9 and the contrast couldn’t be starker. Ismael came into the world as the result of Abraham and Sarah’s scheming and manipulation (Genesis 16). (After all, if being blessed is simply about having the right DNA, there were ways they could control that). But receiving the blessings is about more than genetics, so even though Ishmael had the right DNA, he was not part of Israel (Genesis 17, 21). Rather it was Isaac—the son whose birth came as the result of God’s promise to an unbelieving, barren, old woman—from whom Israel came. And with this, Paul has undercut the central tenet of his objectors by showing that inheriting the promises has always been about more than DNA.
He moves on to Esau and Jacob (v. 10-13) to double down on this point. The promise went to Jacob (who would later become Israel) and was totally independent of his works and solely because of Gods purpose and grace.
2. The children of God are objects of God’s mercy (v. 14-23). It is the children of promise who receive God’s mercy because the bedrock truth is that God did and does things the way He wants (v. 14-18)—not how Israel (or anyone else) thinks He should do things. (We would all do well to remember that).
But we’re grateful for this, aren’t we? Because He always does what is good, right, and true—not because there is some external law He needs to satisfy but because that is who He is! Listen, is anyone upset that God hardened Pharaoh (through giving him repeated opportunities to do the right thing) and showed mercy to Moses by revealing Himself to him (Exodus 33:19)? Of course not! He didn’t override anyone’s choice—He let them do what they wanted to and then (in Pharaoh’s case), use his wickedness for His purposes. Don’t be sacred by the wording of these verses. It’s good news that God is this way. It’s part of what makes God, God.
I think there’s a deeper thought here as well. God is not obligated or indebted to show anyone mercy—Paul has shown in the first three chapters that what all of us deserve is condemnation. Nonetheless, it wouldn’t be fair to say His mercy is given arbitrarily—He’s told us (over and over), that He hardens the proud and lifts up the humble. Yet this was so hard for Israel because they didn’t view themselves as needing God’s mercy—after all—they were the chosen people (see Luke 18:9-14; John 8:31ff)!
The potter Paul speaks of is God and the clay is sinners (all of us). God has the right to use certain sinners as He did Pharaoh, and the right to show mercy to other sinners as He did Moses. To use this text to prove God predestines people to heaven or hell is to take it out of context. All encouragement in Scripture for people making decisions and choosing would be nonsense if this was true (to say nothing of what it would say about God).
3. The children of God are both Jews and Gentiles (v. 24-29).
The Old Testament spoke of this. Paul brings in two texts from Hosea (2:23, 1:10) and uses them to speak of the Gentiles receiving mercy (v. 25-26). And then, just as astonishingly, he speaks from Isaiah (10:22-23, 1:9) of only a remnant being saved (v. 27-29).
4. The children of God are those with faith (v. 30-33).

And how did this unforeseen (though not unpredicted) event come about? Because the Gentiles had faith (v. 30)—something the majority of Israelites did not possess (v. 32-33). We would have led with this point, but in becoming all things to all people, Paul wanted to meet them in their minds by showing them from their Scripture the principles of election.
This also answers the more obvious question for most people reading Romans 9 today—“Why didn’t Paul just tell them they had to embrace Jesus as the Messiah?” Paul is certainly going to talk about Jesus (10:4, 6-7, 9, etc.), but he wanted to do it in the context of election since that was their presenting issue. In all of this though, Paul is pointing them to the truth that if they would have had faith in God, they would have embraced Jesus as the Messiah.