Yes, I'm speaking about v26 which is what the OP is really about right? Of course that's what ch. 11 is about as a whole.
You bring up the olive tree that is in the passage, and it should be brought up in this as context for "all Israel" of v26.
What do we learn from this illustration:
"Rom 11:16 For if the firstfruit is holy, the lump is also holy; and if the root is holy, so are the branches."
We can come back to this, but necessarily the branches are tied closely to the root, as the lump is to the firstfruit. That the branches are holy, as is the root.
"Rom 11:17 And if some of the branches were broken off, and you, being a wild olive tree, were grafted in among them, and with them became a partaker of the root and fatness of the olive tree, "
Branches can be broken off. Even those grafted in (v21-22). Which means if we interpret this as 'all the elect' is Israel - then we can lose our salvation. There is no longer perseverance/preservation of the saints. This is obviously wrong. The branches broken off are 'ethnic Jews' as is the context of the whole passage preceding this, including (9:1-5, 10:1). "His people" v1 Paul is also of them, as an Israelite. He's talking about ethnic Jews. Same v2. V5 from among the same ethnic Jews there is a remnant at the present time (in Paul's day and in our day). V7, Israel (ethnic Jews) has not obtained what it seeks. A small portion have obtained it of those God elected from among ethnic Jews. The rest were blinded by God.
"Rom 11:11 I say then, have they stumbled that they should fall? Certainly not! But through their fall, to provoke them to jealousy, salvation has come to the Gentiles."
They who? Ethnic Jews. They fell/transgressed in order that salvation would come to Gentiles, which by the Gentiles coming would provoke to jealousy the ethnic Jews to also come to salvation.
"Rom 11:12 Now if their fall is riches for the world, and their failure riches for the Gentiles, how much more their fullness!"
Whose fall? The ethnic Jews. Their fall is means for riches to the world/Gentiles. Then how much more will the fullness of the ethnic Jews coming in mean riches for the Gentiles? Even more so. He says the same thing in v15.
Who is Paul speaking to? Gentiles.
"Rom 11:13 For I speak to you Gentiles; inasmuch as I am an apostle to the Gentiles, I magnify my ministry,
Rom 11:14 if by any means I may provoke to jealousy those who are my flesh and save some of them."
Paul is speaking to the Gentiles, and he ministers to Gentiles because he desire the salvation of ethnic Jews, his fellow country men (9:1-5; 10:1). Because at the present time for Paul and us, the gospel is going to the Gentiles and this is the means to bring in the Jews (v11-15).
"Rom 11:21 For if God did not spare the natural branches [ethnic Jews], He may not spare you [Gentiles] either.
Rom 11:22 Therefore consider the goodness and severity of God: on those who fell [v12 ethnic Jews], severity; but toward you [Gentiles], goodness, if you [Gentiles] continue in His goodness. Otherwise you [Gentiles] also will be cut off."
Cut off from the olive tree. That is where perhaps baptists will not get this. The olive tree is clearly the visible church on earth, just as in the illustration of the vine in John 15.
"Rom 11:23 And they [ethnic Jews] also, if they [ethnic Jews] do not continue in unbelief, will be grafted in, for God is able to graft them [ethnic Jews] in again.
Rom 11:24 For if you [Gentiles] were cut out of the olive tree which is wild by nature, and [Gentiles] were grafted contrary to nature into a cultivated olive tree, how much more will these [ethnic Jews], who are natural branches, be grafted into their [ethnic Jews] own olive tree?"
Of which the root is the Patriarchs (v16, 28). The covenant promises communicated to Abraham, Isaac, and Jacob of which all Jews come forth from.
"Rom 11:25 For I do not desire, brethren, that you [Gentiles] should be ignorant of this mystery, lest you [Gentiles] should be wise in your own opinion, that blindness in part has happened to Israel [ethnic Jews] until the fullness of the Gentiles has come in.
Rom 11:26 And so all Israel will be saved..."
The fullness of the Gentiles is of course contrasted by ethnic Jews as it has been even since ch. 1:16 and throughout ch. 11. What is this fullness of Gentiles? It is that a great number of Gentiles coming into the Church, which brings about a great provocation of jealousy to ethnic Jews so ethnic Jews will desire the riches that the Gentiles have (v11, 14). So a partial blindness has happened to [ethnic Jews] Israel (v7). And that for a time. "Until" the fullness of the Gentiles comes in. And in that way, All Israel will be saved. Now who is Israel up until this point? Ethnic Jews. There's no other possibility unless you believe that one can lose their salvation and we rip what Paul is saying out of context (even since 9:1 and for that matter 1:16). But the fact that Paul already has taught what v26 says should convince everyone all the more. He's already said that the riches going to the Gentiles will bring about a fullness of ethnic Jews coming in (v.12, 15). That's why he's ministering to the Gentiles (v.13-14).
Then what? Paul quotes Scripture to prove what he's just said from Isaiah. That God's covennat still stand to "Jacob" - Israel.
Now what else? If you think v.26 suddenly changes from ethnic Jews to all elect Israel (to read like Gal. 6:16) then v28 will disappoint you. The context continues...
"Rom 11:28 Concerning the gospel they are enemies for your sake, but concerning the election they are beloved for the sake of the fathers."
Who is "they"? The context requires that "they" be "All Israel". "All Israel" were enemies for your sake. For whose sake? Gentiles is what context requires. They were enemies to whom? God. He blinded and hardened them. He broke off their branches. All Israel can be nothing but ethnic Jews. But concerning election (v2, 16) they [ethnic Jews] are beloved for sake of the fathers. Who are the fathers? They are the firstfruit, the root --> patriarchs of whom were given the covenant promises.
We continue still.
"Rom 11:30 For as you [Gentiles] were once disobedient to God, yet have now obtained mercy through their [ethnic Jews] disobedience,
Rom 11:31 even so these [ethnic Jews] also have now been disobedient, that through the mercy shown you [Gentiles v11-15] they [ethnic Jews v11, 14] also may obtain mercy."
Who are these people? The same as the beginning of ch. 11. Nothing's changed! The whole context has been ethnic Jews and (ethnic) Gentiles. Paul has from the beginning been concerned about ethnic Jews because he said,
9:1-5 "I tell the truth in Christ, I am not lying, my conscience also bearing me witness in the Holy Spirit,
Rom 9:2 that I have great sorrow and continual grief in my heart.
Rom 9:3 For I could wish that I myself were accursed from Christ for my brethren, my countrymen according to the flesh,
Rom 9:4
who are Israelites, to whom pertain the adoption, the glory, the covenants, the giving of the law, the service of God, and the promises;
Rom 9:5
of whom are the fathers and from whom, according to the flesh, Christ came, who is over all, the eternally blessed God. Amen."
and "Rom 10:1 Brethren, my heart's desire and prayer to God for Israel is that they may be saved."
Paul's concern is for his brethren according to the flesh, that's never changed and so he prays for them and ministers to Gentiles so that Jews will be at some point grafted back into the visible Church on earth and be saved.
Further, Paul is pushing the Gentiles to not boast or be haughty (v16-32). So that Gentiles won't boast of their current position (v18-20). What is going to cause them to not boast? The Jews being grafted back in. If v26 "All Israel" is all the elect, guess what, that doesn't accomplish Paul's purpose of rooting out the pride of the Gentiles over the Jews. So this last thing is to show that Paul's purpose is actually ruined if "All Israel" is not ethnic Jews.
Honestly, I've recently preached on this. Everything I have to say is in the sermons. I posted this in my first post on this thread:
https://www.sermonaudio.com/search.asp?sortby=added&keyword=cpc-kc&SourceOnly=true&subsetcat=bible&subsetitem=Romans&subsetitem2=11&AudioOnly=false