This has to do with national Israel and Gentiles in general being either grafted in or broken off from the blessings of Abraham.
The following quote from a Classic Arminian website and points to how illogical this interpretation is:
Calvinists have traditionally tried to resolve the difficulty in one of two ways. The first way is to say that the branches do not represent individuals, but nations. The broken off branches represent the nation of Israel, and the engrafted branches represent the Gentiles as a people group. The problem with this interpretation is that Paul is speaking of individual branches that have been broken off and grafted in to the true Israel of God. The branches clearly represent individual Jews, for the entire nation has not been rejected. There are believing Jews [the remnant] who have remained in the olive tree. The grafted in branches represent individual Gentiles as only believing Gentiles have come to enjoy the favor and election of God. It is only believing Gentiles that can be called spiritual descendants of Abraham, and it is beyond argument that not all Gentiles have embraced Christ.
arminianperspectives.blogspot.com/search?q=olive+tree
It is also quite clear that the warning about being cut off does
not apply to Gentiles as a whole (believers and unbelievers), but to believing gentiles
only:
**19You will say then, “Branches were broken off so that I might be grafted in.”
20Quite right, they were broken off for their unbelief, but you stand by your faith Do not be conceited, but fear;
21for if God did not spare the natural branches, He will not spare you, either.
22Behold then the kindness and severity of God; to those who fell, severity, but to you, God’s kindness, if you continue in His kindness; otherwise you also will be cut off. **
Since we just had a discussion regarding the use of plural pronouns, how do you explain the use of plural nouns and pronouns in reference to those who were broken off (branch
es, they). How many Jewish nations are there? Secondly, what does Paul say to the Gentiles? He says “you stand by faith.” In other words, he only has
believing Gentiles in view,
not Gentile nations (belivers and unbelievers) as a whole. And what he says regarding being cut off exclsuively applies to Gentile Christians:
but you stand by YOUR FAITH Do not be conceited, but fear
Who is Paul speaking to when he says they should not be conceited, but fear? To those who
believe, because Gentile pagans cannot have faith. And what does he say to Gentile believers,
why must they fear?
**21FOR if God did not spare the natural branches, He will not spare you, either.
22Behold then the kindness and severity of God; to those who fell, severity, but to you, God’s kindness, if you continue in His kindness; otherwise you also will be cut off. **
To reinforce the fact that Paul is speaking to Gentile believers, he says that they must
continue - the Greek word is a variant of the same word used in John 15 - in His kindness, which can only mean he is speaking to individual Gentile
Christians,
not nations in general. And the warning to them is that if they do not continue in His kindness, they will be cut off, suffering the
same fate as unbelieving Israelites.
God bless,
Michael