If I may just make 2 points here :
1. It is a bit of an irony that the passage you quoted concerns Israel - which is called (many times in scripture) God's elect. And most of the Christian world that does not hold Calvinism will at the same time hold that ALL of Israel is elect to salvation (Romans 11:25-26). So those who will reject predestination (at least for Gentiles) will wholeheartedly accept the idea that it will hold for Israel. God, then, can do what He wants with Israel but not with the Gentiles? So which does Paul believe (more...?), that it's up to Israel if they want to believe or not (the chapter you cited) or that they all will be saved per Romans 11 (whether they like it or not?)...
For the record, I don't use that to place undue stress on one or the other passage - just to realize that the arguments against Reformed predestination often seem to be more emotional than biblical. The matter is complex and I don't believe our words can properly do the whole thing justice.
I don't see that as an issue considering the following:
Romans 9:6-8
It is not as though God’s word had failed. For not all who are descended from Israel are Israel. Nor because they are his descendants are they all Abraham’s children. On the contrary, “It is through Isaac that your offspring will be reckoned.” In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring.
Galatians 3:7
Understand, then, that those who have faith are children of Abraham.
2. The second point is one you are no doubt familiar with - the Calvinist doesn't disagree that belief is a necessary component of salvation. And would also say (I suppose) that Paul didn't know who was to be saved or not. Even Jesus spoke what He did first of all out of obedience to the Father, but also hoping the Pharisees "might be saved" (John 5:34). Were they? The Calvinist would say that God alone knows...so Paul's approach wouldn't be inconsistent with a Calvinistic theology (as I understand it, anyway).
I can't see how Calvinist Paul can extricate himself from the charge of disingenuity. The reality would be that Paul is merely covering the election doctrine with some flowery words. One can feel Paul's genuine concern in his desire for their salvation - such a contrast to the stark theology of TULIP.
Why quote from Moses to the effect that it's not too difficult? TULIP says it wont happen unless you're 'in'. Why enjoin belief in something that some folk have been excluded from? How can the Calvinist preacher do this? How could J. Piper
do so when we know he doesn't believe Christ died for all
salvifically?
When the Gospel writers recorded the events concerning Jesus Christ and the Passion, they endeavoured to do so accurately - as Luke says at the beginning of his:
Many have undertaken to draw up an account of the things that have been fulfilled among us, just as they were handed down to us by those who from the first were eyewitnesses and servants of the word. With this in mind, since I myself have carefully investigated everything from the beginning, I too decided to write an orderly account for you, most excellent Theophilus, so that you may know the certainty of the things you have been taught.
Paul's recourse for his kinsmen appears to be anything but genuine under Calvinism.