What does Romans 15:8 mean?
Paul is addressing Christ's unique relationship with the Jews and the Gentiles. First, Christ, during His time on earth and continuing even now, became a servant to the circumcised. Israel's identity was closely associated with circumcision. Christ, then, became in His earthly life and ministry a servant to the Jewish people.Jesus said something similar in Matthew 15:24, "I was sent only to the lost sheep of the house of Israel." Paul echoes this idea about Jesus' purpose in Galatians 4:4–5, "God sent forth his Son, born of woman, born under the law, to redeem those who were under the law."
It's not that Jesus' work on earth did not also benefit non-Jewish people, as the following verse will state clearly. It's that in Jesus, God was keeping all His promises to Abraham and the patriarchs. In sending Jesus as the Messiah, God was proven to be a keeper of His promises to Israel.