Romans 9:25-26 — As He saith also in Hosea, I will call them My people, which were not My people; and her beloved, which was not beloved.
And it shall come to pass, that in the place where it was said unto them, Ye are not My people; there shall they be called the children of the living God.
Quote in verse 25 — And I will sow her unto me in the earth; and I will have mercy upon her that had not obtained mercy; and I will say to them which were not My people, Thou art My people; and they shall say, Thou art my God (Hosea 2:23).
Quote in verse 26 — Yet the number of the children of Israel shall be as the sand of the sea, which cannot be measured nor numbered; and it shall come to pass, that in the place where it was said unto them, Ye are not My people, there it shall be said unto them, Ye are the sons of the living God (Hosea 1:10).
Israel rejected God and became "not My people," so God, when He recalls the remnent will also call Gentiles who are also "not My people." But these verses refer to Israel.
Paul is showing from Hosea that "they are not all Israel which are of Israel."
Many commentators apply verses 25-26 to Gentiles being saved today, but an examination of the Old Testament passages reveals that they refer rather to redeemed Israel as Jehovah's reconciled wife. He had given her "a bill of divorcement" (Isaiah 50:1) and had called her "Lo-ammi," "Not My People" (Hosea 1:9-10) because of her spiritual adultery, but He never ceased loving her or abandoned His promise and purpose to make her His own indeed. This will take place after God has "finished the work and cut it short in righteousness" (verse 28), i.e., after He has judged Israel and the nations in the great tribulation. — The Epistle of Paul to the Romans, by Cornelius R. Stam, page 233.
(See also God's Covenants, by Donald Grey Barnhouse, pages 52-54.)