Lest you be wise in your own sight, I do not want you to be unaware of this mystery, brothers: a partial hardening has come upon Israel, until the fullness of the Gentiles has come in. (ESV)
You have probably heard someone say, "God's time is not our time." This is true. God is a sovereign God. He does things according to his perfect eternal plan. Consider the timing of Christ's entrance into humanity. Ephesians 4:4 describes the timing of Christ's birth as, "When the fullness of time had come." God's timing is perfect.
The Israelites are experiencing a partial hardening of their hearts until the fullness of the time of the Gentiles. This hardening started with the Jews to a large degree rejecting Jesus as their Messiah. Not every Jew rejected the Gospel. Paul himself was a Jew who gave his life to Jesus Christ. God has always kept for himself a remnant of believers.
We do not necessarily know how long this partial hardening of the Jews will last. One of the things that I think is important is that all the Gentiles who have given their lives to Jesus Christ should demonstrate kindness to the Jews. While they may not be coming to Jesus in large quantities right now, there are Jews who are coming to Jesus as Messiah. We need to keep proclaiming the truth.
In this passage of Scripture Paul is concerned about how the Gentiles treat the unbelieving Jews. There is no place for Gentiles looking down their noses at the Jews who have not believed. While the Jews' hearts may be hardened at the moment God has a time of harvest where there will be many more Jews coming to him. It is all in God's timing. God's way is perfect. So is his timing.
Let us all continue to pray for the Jewish people that God would soften their heart and they would receive Jesus as their Messiah. While we wait, we need to have hearts of compassion.