Romans 9:25-29

25 As indeed he says in Hosea,
“Those who were not my people I will call ‘my people,’
and her who was not beloved I will call ‘beloved.’”
26 “And in the very place where it was said to them, ‘You are not my people,’
there they will be called ‘sons of the living God.’”

27 And Isaiah cries out concerning Israel: “Though the number of the sons of Israel be as the sand of the sea, only a remnant of them will be saved, 28 for the Lord will carry out his sentence upon the earth fully and without delay.” 29 And as Isaiah predicted,

“If the Lord of hosts had not left us offspring,
we would have been like Sodom
and become like Gomorrah.”

Throughout this whole section, Paul has been dealing with four related questions:

1. Why did so many of the Jews refuse to accept Jesus as Messiah?
2. In light of Jewish rejection of Jesus and Gentile acceptance of Jesus, has God replaced the Jews with the Gentiles as His chosen people?
3. Is there any hope that the Jews who previously rejected Jesus as Messiah might one day turn back and believe?
4. In light of all of the above, what is God up to?

There are more Old Testament quotations in Romans 9–11 than in the whole rest of the letter, and here Paul offers a few more.

First, from Hosea, Paul makes a point about the Gentiles:

25 Those who were not my people I will call ‘my people,’
    and her who was not beloved I will call ‘beloved.’”
26 “And in the very place where it was said to them, ‘You are not my people,’
    there they will be called ‘sons of the living God’” [Romans 9:25–26].

Paul uses the quotation from Hosea to make the point that the Lord was always planning to bring in people into His family who were originally outsiders, so Paul wants the Romans to understand that the inclusion of the Gentiles shouldn’t be shocking.

Second, from Isaiah, Paul makes a point about the Jews:

27 And Isaiah cries out concerning Israel: “Though the number of the sons of Israel be as the sand of the sea, only a remnant of them will be saved, 28 for the Lord will carry out his sentence upon the earth fully and without delay.” 29 And as Isaiah predicted,
“If the Lord of hosts had not left us offspring,
we would have been like Sodom
and become like Gomorrah” [Romans 9:27–29].

Just as the Old Testament prophets has said that the Lord would one day bring in the Gentiles, Paul also reminds the Roman church that the Lord had warned the Jews that there would be consequences for unbelief:

1. In 9:27 Paul quotes Isaiah to make the point that just because there are lots of people related to Abraham does not mean they will all be saved; rather, only the small (faithful) number will be saved;

2. In 9:29 Paul quotes Isaiah to make the point that, even if the majority of Israel rejects the Messiah, nevertheless there will still be a faithful remnant.

Paul’s point is just to explain to the Roman church that the inclusion of the believing Gentiles and the exclusion of unbelieving Jews was already foretold in Scripture.