What does Romans 9:28 mean?

Paul is in the middle of a quote from Isaiah 10:22–23 about the sons of Israel. He is showing that even in the Old Testament era, God was clear that only a remnant of the Israelites would be saved, not the entire offspring of that nation. In verse 24, Paul wrote that God has called out His people from both the Jews and the Gentiles. He has called them to faith in Christ and to receive His mercy.

Why do we need mercy from God that is only available in Christ? That is answered here in the quote from Isaiah: judgment is coming. The Lord will carry out his sentence on the earth for the sins of humanity. It will come quickly and without delay, in terms of God's timing. In other words, when the moment for judgment comes, God will not hesitate.

This is a startling thing to some who have grown up Jewish and under the law. As Paul showed in Romans chapter 2, many Jews of his era believed they would not face God's judgment simply because they were born Jewish. Paul, though, makes clear that all who are not in Christ—including those who have not been called out from among the Jewish nation to be a remnant of the Jewish people—will receive God's wrathful justice.