Let’s take the entire statement from Romans 9:15-18:
For He says to Moses, “I will have mercy on whomever I will have mercy, and I will have compassion on whomever I will have compassion. So, then it is not of him who wills, nor of him who runs, but of God who shows mercy. Therefore He has mercy on whom He wills, and whom He wills He hardens.
After reading the above, doesn’t it naturally follow that you would ask: upon whom has God chosen to give mercy, and who has God chosen to harden? Why is it that I don’t find people asking this question? If God said, “I will give a million dollars to whomever I will”, then won’t people be asking the question: “to whom will God give a million dollars”?
Upon whom did God choose to give mercy? The answer is found in the very same letter in Romans 11:30-32:
For as you [Gentiles] were once disobedient to God, yet have now obtained mercy through their [Jews'] disobedience, even so these also have now been disobedient, that through the mercy shown you they also may obtain mercy. For God has committed them all to disobedience, that He might have mercy on all.
The Gentiles obtained mercy. Mercy was shown to the Gentiles. Apparently, God willed to show mercy to the Gentiles.
Who did God choose to harden? The answer to this question is found in Romans 11:7:
What then? Israel has not obtained what it seeks; but the elect have obtained it, and the rest were blinded.
The vast majority of Jews were blinded to the Gospel - hardened, if you will. Apparently, God willed to harden the Jews.
What errors do you find in the approach I have taken here? Thanks.