God chose the Jews(Israelites) as His people. All other peoples were called Gentiles, who had to become Jews(not genetically, but in practice) in order to have a relationship with God.
Finally, God came to earth, died on the cross, and rose again for 3 reasons: (1),the Jews would no longer have to regularly sacrifice lambs, because Jesus Christ was the perfect sacrifice, (2),the Gentiles could now have personal relationships with God, without having to become Jews and be circumcised, and (3),to fulfill prophecy.
All the Jews who lived before Jesus' life, and believed in the prophecies about His life, death, and resurrection were saved through their faith. Now, all Jews and Gentiles who look back on Jesus' life and believe that He was the Messiah, who came to die and rise from the dead, are Christians, literally meaning "Christ-followers". Some Jews, however, believe that Jesus was not the Messiah, and that the Christ has yet to come. These Jews are not Christians, and continue in the ancient Jewish customs.