God's master plan encompasses all of human history. Initially, Israel was to be a light unto the Gentiles. But after Yeshua's death and resurrection the roles were somewhat reversed. It is now the responsibility of the Gentiles to provoke Israel to jealousy.
Romans 11:25 (KJV)
For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
First off it is important to notice that Paul has given the Gentiles a warning, that they are not to think themselves wise. God was the one who had placed the blindness and allowed the Gentiles to come in. Any thinking on the Gentiles' part that their understanding and acceptance of Messiah is due to their being better than the Jews is arrogance and is sinful.
Secondly, the blindness that has been placed over Israel's eyes will last "until the fullness of the Gentiles be come in." But what does this mean? Many have believed that it refers to a specific number of Gentiles that are added to the Kingdom of God. Others have believed that it refers more to the quality or maturity level of the Gentiles. This is probably more likely. But the determining factor seems to be the provoking of Israel. When this is complete the blindness will be removed.
What happens when the blindness is gone?
Romans 11:26 (KJV)
And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:
All Israel will be saved! At some point in time the nation of Israel as a whole will realize that Yeshua is indeed her Messiah. She will accept Him and He will deliver her from her sin.
Zechariah 12:10 (KJV)
And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplications: and they shall look upon me whom they have pierced, and they shall mourn for him, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his firstborn.
This will be a glorious time!
But a question remains. By God's placing a blindness on the Jews has He allowed countless Jews to die in their sin, condemning them to Hell? The answer is "no". Throughout history God has accepted any Jew or Gentile who has come to Him in faith. Salvation has always been available by faith to anyone who sought the God of Abraham, Isaac, and Jacob. The truth of the matter is that most human beings reject Yeshua. But there has always been a remnant of true believers throughout history. When Paul spoke about all Israel being saved he was referring to the remnant of Israel that survives the tribulation and then as a nation is saved.
But what an amazing thought. Human beings who are so fallen, can through faith participate in the Kingdom of God, whether they are Jewish or Gentile. Praise God!