You need to use the entire Bible as your context.
Being grafted into or cut off from the olive tree is NOT talking about salvation and eternal life.
Romans 11 is all about the FALL of Israel and their future restoration.
Paul says it over and over again:
Rom 11:11-12 KJV I say then, Have they stumbled that they should fall? God forbid: but rather through their fall salvation is come unto the Gentiles, for to provoke them to jealousy. (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?
I don't know how he could make it more clear.
Rom 11:13-15 KJV For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, I magnify mine office: (14) If by any means I may provoke to emulation them which are my flesh, and might save some of them. (15) For if the casting away of them be the reconciling of the world, what shall the receiving of them be, but life from the dead?
Again and again....
Rom 11:25-27 KJV 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. (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: (27) For this is my covenant unto them, when I shall take away their sins.
Crystal clear.