Well then give me Arnold's commentary on this verse:
"For if their having been cast aside has carried with it the reconciliation of the world, what will their being accepted again be but Life out of death?" (Ro.11:15).
Though he starts out rightly asserting that the Olive Tree represents access to blessing - which is accessed by faith - he then goes off into the error others espouse - that Romans 11 is relating that the Gentiles have been made partakers with the Believing Remnant of Israel of the blessing said Olive Tree represents.
Thing is, the Tree represents access to blessing in general: the particular economy determining what said blessing is and or how it is accessed by faith.
As when Abraham believed God 430 years before the Law and his faith alone was reckoned for righteousness.
To that was then added, circumcision as a condition of said access, then the Law...
But now, access by faith is once more without the works of the Law, as He hath concluded all under sin through the Law, that He might have mercy upon all without distinction.
The Tree still represents access, but right now it is not Israel's blessings that it represents.
These blessings here...
Romans 9:3 For I could wish that myself were accursed from Christ for my brethren, my kinsmen according to the flesh: 9:4 Who are Israelites; to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and the promises; 9:5 Whose are the fathers, and of whom as concerning the flesh Christ came, who is over all, God blessed for ever. Amen.
That was put on hold until this here...
Romans 11:12 Now if the fall of them be the riches of the world, and the diminishing of them the riches of the Gentiles; how much more their fulness?
Romans 11:25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in. 11:26 And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob: 11:27 For this is my covenant unto them, when I shall take away their sins. 11:28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes. 11:29 For the gifts and calling of God are without repentance.
In the meantime, the mercy is now Gentile...
Romans 11:30 For as ye in times past have not believed God, yet have now obtained mercy through their unbelief: 11:31 Even so have these also now not believed, that through your mercy they also may obtain mercy. 11:32 For God hath concluded them all in unbelief, that he might have mercy upon all.
Romans 3:9 What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin; 3:10 As it is written, There is none righteous, no, not one: 3:11 There is none that understandeth, there is none that seeketh after God.
3:19 Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God. 3:20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin. 3:21 But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets; 3:22 Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: 3:23 For all have sinned, and come short of the glory of God; 3:24 Being justified freely by his grace through the redemption that is in Christ Jesus: 3:25 Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; 3:26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus. 3:27 Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith. 3:28 Therefore we conclude that a man is justified by faith without the deeds of the law. 3:29 Is he the God of the Jews only? is he not also of the Gentiles? Yes, of the Gentiles also: 3:30 Seeing it is one God, which shall justify the circumcision by faith, and uncircumcision through faith. 3:31 Do we then make void the law through faith? God forbid: yea, we establish the law.