What does Romans 9:27 mean?

Paul is showing that God shows mercy to whomever He will, including some Jews and not others, as well as to some Gentiles. God has declared that the deciding factor about who will be spared from His wrath and receive His mercy is by faith in Christ, not human action (Romans 6:23). God's choice in how, when, or if that happens for any particular person is a decision He can make according to His own sovereign purposes (Romans 9:20–21).

In the previous verse, Paul used Hosea's words to describe how some Gentiles have now been called by God and included as His beloved children. Now Paul quotes from Isaiah 10:22–23, regarding the Jewish people.

First, God has kept His promise to Abraham. The sons of Israel have become as the sands of the sea (Genesis 22:17). That promise did not obligate God, however, to eternally save every son and daughter of Israel. In fact, Isaiah writes that only a remnant will be saved.

This and the following verses bring us back around to where Paul began this chapter. God does and will continue to keep His promises to His chosen nation Israel. However, He will not show the same mercy to every descendant of Abraham, as many of the Jews assumed. They must come to Him through faith in Christ to be saved from the penalty of their sin, and a remnant of Israelites will do just that.