Romans 11
HARDEST SCRIPTURE EVER!
God did not reject the whole Jew race.
Gentiles get the grace because Jews rejected the gospel. So the gospel come from Jews, if Jews has accepted God's gospel at the first place, we would not have the chance to be saved.
'but God's kindness to you, provided you continue in his kindness. Otherwise you too will be cut off. And even they, if they do not continue in the disbelief, will be grafted them in again.' is not talking about the gentile Christians who has already got the grace will lose it if he loses his faith. But talking about the Jew race and gentiles rather than persons. The grace comes to gentiles because the Jew's disbelief, if gentiles do not believe too, God will remove his grace just like how he removed the grace to Jews.
The way how the grace works on Jews and gentiles are beyond human's understanding. Proving God's wisdom and intelligence is not reachable by human.