Don't know how an Arminian would answer, but I always go to the OT verses that Paul quotes because Paul says that's what noble Bereans do (Acts 17:11).
Paul quotes Hosea in regards to those who were at one time called "not my people" and receiving "no mercy" would one day be called "my people" and would have "mercy".
Romans 9
(25) As indeed he says in Hosea, “Those who were not my people I will call ‘my people,’ and her who was not beloved I will call ‘beloved.’”
(26) “And in the very place where it was said to them, ‘You are not my people,’ there they will be called ‘sons of the living God.’”
Or another way to put it, God used the same lump of clay to be rebellious and then be restored to non-rebellious.
Paul continues this line of thought on into chapter 11.
Romans 11
(30) For just as you were at one time disobedient to God but now have received mercy because of their disobedience,
(31) so they too have now been disobedient in order that by the mercy shown to you they also may now receive mercy.
(32) For God has consigned all to disobedience, that he may have mercy on all.
Paul already explained in previous verses (shown below) that Israel's temporary blindness and fall would open the door to Gentiles (who in the OT were also "not my people") and could now be restored as "my people" just as Israel could be restored as "my people".
Romans 11
(11) So I ask, did they stumble in order that they might fall? By no means! Rather, through their trespass salvation has come to the Gentiles, so as to make Israel jealous.
(12) Now if their trespass means riches for the world, and if their failure means riches for the Gentiles, how much more will their full inclusion mean!
