But their actions are based on God granting them grace. If God has not granted them grace then they cannot be grafted back in.
Does Paul know which God has done. . .so how would you have him state it?
There is only one way to know and one way to state it, based on their actions.
So the determining factor is not their actions but God’s action according to Calvinism.
Right. But men's actions are the revealing factor showing what God has done, rebirth or not, and therefore their actions reveal their destiny, heaven or not.
Not to mention that the Gentiles who were grafted in by God can be cut off. Why would He graft people in according to His foreknowledge then cut them off?
You think God was caught by surprise when he made Israel his people, and they then rejected his Messiah, having him crucified?
So God had to go to Plan B?
You think that all happened apart from God's will?
Does God consider everyone his son just because he has been baptized, if his lifestyle is consistent disobedience and unbelief?
Being in the "community of God's people" does not make you a son of God, and you are subject to "cutting off" if you are not his son.
They were grafted in as God's people (the church, assembly in the desert, Acts 7:38) and then cut off when they rejected the Messiah.
Therefore, the Gentiles were made God's people, the church (called-out assembly).
Did God grant these Gentiles grace then revoke it?
What grace?
Does God grant grace to someone just because he occupies a pew at church?
Grace is not a reward. Grace saves. If your actions reveal you are not saved, you have been given no grace and, therefore, no grace is revoked.