As Paul had laid out God's plan for believers, his heart was stung with pain over his fellow Jews that rejected Messiah. Paul concluded that not all Israel was Israel! God's Word had not failed, but was He unjust in His choices? God had preferred Isaac over Ishmael and Jacob over Esau. Paul continued to explain.
Romans 9:15-24 (KJV)
For he saith to Moses, I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion.
So then it is not of him that willeth, nor of him that runneth, but of God that sheweth mercy.
For the scripture saith unto Pharaoh, Even for this same purpose have I raised thee up, that I might shew my power in thee, and that my name might be declared throughout all the earth.
Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth.
Thou wilt say then unto me, Why doth he yet find fault? For who hath resisted his will?
Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed it, Why hast thou made me thus?
Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?
What if God, willing to shew his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction:
And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory,
Even us, whom he hath called, not of the Jews only, but also of the Gentiles?
In verse 15 Paul reminded his readers that God had told Moses that He would have mercy and compassion on those of His choosing.
Exodus 33:19 (KJV)
And he said, I will make all my goodness pass before thee, and I will proclaim the name of the LORD before thee; and will be gracious to whom I will be gracious, and will shew mercy on whom I will shew mercy.
Verse 16 further relates that the choice is not because of the one who might will the choice, or the one who might run after or try to do something for the choice, but rather it is only up to God.
For example, the reason that Pharaoh was raised up was so that God's power and might would be declared throughout all the earth (verse 17). God hardened Pharaoh's heart and gave mercy to the Israelites (verse 18).
The natural question then, is, "Why does God find fault (verse 19)?" If God's will is sovereign, isn't He responsible for Pharaoh's actions and not Pharaoh? In like manner, isn't it God's fault that many of the Jews have rejected the Messiah?
Paul answered negatively (verse 20). What right does does mankind have to reply against God? What right would a created being have to question the intent of the Creator?
The potter has power over the clay (verse 21). The potter can choose to make one lump of clay either honorable or dishonorable.
Paul's discussion highlighted God's right as Creator to do whatever He chose with His creation. He is sovereign. Yet, Paul is not unmindful of the tension between God's sovereignty and man's free will. In chapters 1 and 2 of Romans Paul explicitly stated that mankind is without excuse as to their sin. They are responsible. Then in looking forward, Paul stated that WHOSOEVER calls upon the name of God will be saved.
Romans 10:13 (KJV)
For whosoever shall call upon the name of the Lord shall be saved.
Although God is sovereign, "God does not harden the heart of anyone but a confirmed rebel; He wants all to turn from sin to Him" (David H. Stern. Jewish New Testament Commentary, Jewish New Testament Publications, Inc., Clarksville, MD, 1992. p. 391).
Then in verse 22 Paul shifted gears slightly by pointing out God's long suffering in enduring those vessels of wrath (as in the potter's creations) by giving them time to repent. In the same way, God makes known the riches of His glory on the vessels of mercy, which have been prepared unto glory (verse 23).
In verse 24 Paul revealed that those vessels of mercy are those of the Jews that have been called. With this statement Paul must have sighed in his heart. Although not all the Jews accepted Messiah, there was a called remnant that responded to God's mercy. Plus, there were also Gentiles who responded and embraced Messiah.
So who is Israel? The true Israel is made up of both Jews and Gentiles who have responded in repentance and faith to Yeshua the Messiah.