And in them the prophecy of Isaiah is fulfilled, which says: “Hearing you will hear and will not understand, and seeing you will see and not perceive; For the hearts of this people have grown dull. Their ears are hard of hearing, and their eyes they have closed, lest they should see with their eyes and hear with their ears, lest they should understand with their hearts and turn, so that I should heal them.’ – Matthew 13:14-15

For I do not desire for you to be ignorant, brethren, of this mystery in order that you are not alongside yourselves in a frame of mind, because hardness from a part to Israel has come until which the fullness of the Gentiles has come. – Romans 11:25

The nation of Israel has been blinded due to their unbelief for a short time while the Gentiles are being offered reconciliation. God uses an example of a cultivated olive tree to show that the Gentiles are now able to partake in the benefits that belong to Israel, but due to her unbelief she has been cut off until the fullness of the Gentiles comes in.

Therefore, you say, “Branches were broken off in order that I on my part should be grafted in,” Well, by unbelief they were broken off, and you by faith stand not having a high frame of mind but respect. – Romans 11:19-20

The placement of sons, expressing a proper opinion of God, the covenants, the legislation, the religious service, and the promises belong to Israel. The Church does not replace Israel in these, but rather is allowed to partake in the benefits of being in Christ.

For I vow for myself that I myself were separated from the Christ on behalf of my brethren, my kinsmen according to the flesh, who are Israelites, to whom [is] the placement of sons and the glory and the covenants and the legislation and the religious service and the promises, of whom are the fathers and out from whom the Christ, according to the flesh, the one who is over all, God well-spoken of into the ages, amen. – Romans 9:3-5

Yes, Israel as a nation has been cut off; however, those individual Jews that believe in the death concerning sins and resurrection of Christ are grafted back in and therefore partake of the benefits of being in Christ along with the Gentiles.

and those ones also, if perhaps they do not remain in unbelief, they will be grafted in. For God has the natural ability to again draft them in. – Romans 11:23

The Gentiles do not take the place of Israel, and therefore neither does the Church. For God will save Israel, the nation, because He made a promise to do so.

And thus all Israel will be saved just as it stands written, “The Deliverer will come out of Zion and turn away the ungodliness of Jacob.” – Romans 11:26

The Mystery of the Blindness of Israel is that the Gentiles are now being reconciled to God because of her unbelief. However, this blindness will only last until the fullness of the Gentiles comes in, at which time Israel will see the Messiah, Jesus, for Who He truly is and will be saved. Therefore, do not be ignorant of this mystery or high minded against Israel, for she belongs to God and He will save her.