Verse 19 of Romans 9 begins an aside in Paul’s dissertation. He has a hypothetical responder object to the repercussions of Paul’s presentation, arguing that if all depends on God’s mercy, than those hardened cannot be blamed for their hardness. This, of course, presages Paul’s revelation in chapter 11 that currently, a hardening has come upon the Jews. Hardness is not presented there as a necessarily persevering quality, so the response is more directed at the hypothetical questioner’s arrogance than at establishing any principle of deterministic election.
The principle of Romans 9 is not that God arbitrarily chooses certain individuals to be saved and others to be damned. Instead, it teaches that anyone who receives God’s promise by faith and relates to God on the basis of his mercy, rather than works, will be part of his people. God is God and can do as he likes, but if Gentiles, regardless of lacking the Jews' “God-givens,” can be made righteous by faith, anyone can. And by God, you too, whoever you may be, can.
Parts I, II