My friend in Jordan, you have asked a wonderful question that would normally require many pages to answer. I will try my best to condense my response.
Both Scripture and history provide answers to this question. In the Injil according to John, chapter 1, verse 11 and 12, we read "He came to that which was his own, but his own did not receive him. Yet to all who received him, to those who believed in his name, he gave the right to become children of God."
The Scripture further reveals that one of the great prophets of the first century A.D., Paul the apostle struggled with this same questions. Paul speaks clearly in Ephesians 3:2 about the mystery made known to him by revelation. Paul identified that mystery in verse 6 when he says, "This mystery is that through the gospel the Gentiles are heirs together with Israel, members together of one body, and sharers together in the promise in Christ Jesus (Isa Al-Masih)." And in verse 8 he continues, "Although I am less than the least of all God's people, this grace was given me; to preach to the Gentiles the unsearchable riches of Christ (Al-Masih)." Of course, in general the word "Gentiles" simply refers to those who are not of Bani-Israel.
Part of the "revelation" that Paul the Apostle refers to in Ephesians 3:2 came to him as he traveled from Jerusalem to Damascus. In the Acts of the Apostles 26:13-14 the Scripture records that Paul fell to the ground as the result of a light from heaven. And in verses 15 to 18, it says that he heard the following words: "I am Jesus (Isa), whom you are persecuting….. Now get up and stand on your feet. I have appeared to you, to appoint you as a servant and as a witness of what you have seen of me and what I will show you. I will rescue you from your own (Jewish) people and from the Gentiles. I am sending you to them to open their eyes and turn them from darkness to light, and from the power of Satan to God, so that they may receive forgiveness of sins and a place among those who are sanctified by faith in me."
The Apostle Paul was so convinced that Isaiah had come to all humankind that in Romans 10:11-13 he said "…..the Scripture says, "Anyone who trusts in him will never be put to shame; for there is no difference between Jew and Gentile - the same Lord is Lord of all and richly blesses all who call on him for "Everyone who calls on the name of the Lord will be saved…"
Second, history itself shows us that Isa Al-Masih did not come only to Bani-Israel. While for the most part the Jewish people have rejected Isa Al-Masih, millions and millions of Gentiles have discovered his "unsearchable riches.".