I have a question from one of my students. I know half the answer, but he did bring up an interesting question: if the Jews weren't provoked to jealousy and didn't reject God, would the Gentiles have received salvation? Even though from the prophets writing it was decided a long time ago the Gentiles would be God's people. It was just an interesting question and I thought I would ask. Even though I know the Jews probably rejected God on their own without the help of God. They're that type of people or nation. God said so constantly.
"What then? That which Israel seeks for, that he didn't obtain, but the chosen ones obtained it, and the rest were hardened. According as it is written, "God gave them a spirit of stupor, eyes that they should not see, and ears that they should not hear, to this very day." David says, "Let their table be made a snare, and a trap, a stumbling block, and a retribution to them. Let their eyes be darkened, that they may not see. Bow down their back always." I ask then, did they stumble that they might fall? May it never be! But by their fall salvation has come to the Gentiles, to provoke them to jealousy. Now if their fall is the riches of the world, and their loss the riches of the Gentiles; how much more their fullness? For I speak to you who are Gentiles. Since then as I am an apostle to Gentiles, I glorify my ministry; if by any means I may provoke to jealousy those who are my flesh, and may save some of them. For if the rejection of them is the reconciling of the world, what would their acceptance be, but life from the dead?" (Romans 11:7-15).
I think you misunderstood this passage slightly. Paul originally poses the question, "I ask then, did God reject his people?" (Romans 11:1), referring to the Jews. Paul's initial response was basically, "I certainly hope that it will never be the case," because, after all, he was a Jew by birth.
Rather what happened is that the majority of the Jews rejected Christ. Thus the salvation the Jews, in general, sought wasn't obtained. But their fall doesn't imply that God gave up on them. Their fall was a step God used to bring salvation to all of mankind, not just the Jews because it was the Jews' rejection of Jesus that led to his death on the cross and our salvation from sin. "And I, if I am lifted up from the earth, will draw all peoples to Myself" (John 12:32).
God then turned around to use the jealousy that the Jews felt toward the Gentiles who became His people to spur them out of their complacency and back into a relationship with Him. This was foretold in the parable of the prodigal son. Do you remember the reaction of the older brother? "But he was angry and would not go in. Therefore his father came out and pleaded with him" (Luke 15:28). Whether the elder son entered is not stated in the parable because the choice was the Jews to make. Paul states that it worked to save a few more, though it wasn't a wholesale return to God.
Thus, to answer your question, if the Jews had not rejected Jesus, then Jesus would not have died on the cross and none of us would have been saved from our sins -- including the Jews.
See: "What if the Jews had Accepted Jesus?" for more on this topic.