From the discontinued Bob Hill site Biblical Answers:
Next, let’s look at the most important passage on adoption concerning the body of Christ, Eph 1:4-6. Just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love, 5 having predestined us to adoption as sons by Jesus Christ to Himself, according to the good pleasure of His will, 6 to the praise of the glory of His grace, by which He has made us accepted in the Beloved.
Corporate Israel was God’s son by adoption according to Romans 9:4. We also see that the nation will not be cast away. But the nation will be purged in the tribulation according to
Zec 13:8,9 And it shall come to pass in all the land, says the Lord, that two-thirds in it shall be cut off and die, but one- third shall be left in it: 9 I will bring the one-third through the fire, will refine them as silver is refined, and test them as gold is tested. They will call on My name, and I will answer them. I will say, “This is My people”; And each one will say, “The Lord is my God.”
This will be in fulfillment of Hos 1:6-11.
And she conceived again and bore a daughter. Then God said to him: Call her name Lo-Ruhamah, For I will no longer have mercy on the house of Israel, But I will utterly take them away. 7 Yet I will have mercy on the house of Judah, will save them by the Lord their God, and will not save them by bow, nor by sword or battle, by horses or horsemen. 8 Now when she had weaned Lo-Ruhamah, she conceived and bore a son. 9 Then God said: Call his name Lo-Ammi, for you are not My people, and I will not be your God. 10 Yet the number of the children of Israel Shall be as the sand of the sea, which cannot be measured or numbered. And it shall come to pass in the place where it was said to them, You are not My people, There it shall be said to them, You are sons of the living God. 11 Then the children of Judah and the children of Israel Shall be gathered together, and appoint for themselves one head; and they shall come up out of the land, for great will be the day of Jezreel!
But only believing Israel will be saved.
Rom 11:2,24,25 God has not cast away His people whom He foreknew. Or do you not know what the Scripture says of Elijah, how he pleads with God against Israel, saying, 25 For I do not desire, brethren, that you should be ignorant of this mystery, lest you should be wise in your own opinion, that blindness in part has happened to Israel until the fullness of the Gentiles has come in. 26 And so all Israel will be saved, as it is written: The Deliverer will come out of Zion, And He will turn away ungodliness from Jacob.
Every member of the body of Christ has the adoption based on God’s predestination, because we have all put our trust in Christ. We can’t lose our adoption. The body of Christ was predestinated to receive this adoption. We became members of this body and its blessings when we believed.