The Apostle Paul wrote about a time where Israel’s heart would be hardened until “the fullness of the Gentiles,” so what does that mean, and when will that time come?
Salvation is of the Jews
Jesus told the Samaritan Woman at the Well, “You worship what you do not know; we worship what we know, for salvation is from the Jews” (John 4:22). So what did Jesus mean that “salvation is from the Jews?” First of all, the Jews are the holder of the oracles of God, or the Scriptures. They’ve managed to preserve the Old Testament, and today we have the New Testament, of which 39 of the 40 authors are Jewish. Mark is the only exception, but most scholars understand that it’s actually the Apostle Peter’s rendition of Jesus’ earthly ministry and John Mark simply wrote down these experiences. Even so, as with all Scriptures, Mark was under the inspiration of the Holy Spirit when he wrote the Gospel of Mark, because all the authors of Scripture are inspired by the Holy Spirit (2 Pet 3:16), but the Jews were the first to believe in Jesus. In the Old Testament, Abraham believed God and God accredited that to him as righteousness (Gen 15:6; Rom 4:3), so he too was saved. The Apostle Paul wrote, “They are Israelites, and to them belong the adoption, the glory, the covenants, the giving of the law, the worship, and the promises” (Rom 9:4), so it’s not surprising that when Jesus sent out the disciples as evangelists, He told them to only go “to the lost sheep of the house of Israel” and not to the Gentiles (Matt 10:6). Jesus said, “I was sent only to the lost sheep of the house of Israel” (Matt 15:24), but in time, the gospel would go to the Gentiles, as we read in the Book of Acts.
Israel’s Hardness of Heart
Even today, the Jews generally do not accept Jesus Christ as the fulfillment of the prophesied Messiah. Fewer still have put their trust in Christ, having been born again, so today, it’s like a veil has covered their hearts and they are not able to believe and receive the gospel. Paul said of Israel that “to this day whenever Moses is read a veil lies over their hearts” (2 Cor 3:15). And “to this day,” the Jews as a nation still do not believe in Jesus Christ and are not open to trusting in Him…but that day is coming. It must have grieved Paul that his fellow Jews would not believe in Christ because “their minds were hardened. For to this day, when they read the old covenant, that same veil remains unlifted, because only through Christ is it taken away” (2 Cor 3:14), but the Word of God also tells us that day is coming. Just when that day will be, we cannot say for certain.
The Fullness of the Gentiles
The veil in the temple of God separated the Holy of Holies from the rest of the temple. Basically, it was a very large and heavy curtain that was thick enough to prevent someone from accidentally falling into the Holy of Holies, resulting in instant death, but today, a veil covers the hearts of most Jews, and really, all who don’t believe the gospel. The Apostle Paul wrote that “a partial hardening has come upon Israel, until the fullness of the Gentiles has come in” (Rom 11:25b). Notice that it’s only a “partial hardening” and not a permanent one or a fully hardened heart. That means there is still hope for the House of David, so God is definitely not finished with His people. When Paul uses the phrase “the fullness,” in referring to time, he means the exact moment when it will happen, as with Jesus’ arriving as God in the flesh, being born of a virgin. His birth was part of God’s divine providence. The time and place of His birth was fixed, thousands of years before it happened. With respect to Jesus’ coming in the flesh, Paul says that “when the fullness of time had come, God sent forth his Son, born of woman, born under the law” (Gal 4:4). The “fullness” means there was a very specific point in time and history for Jesus to be born, and the day is coming when the Jews hearts will no longer be hardened because God will remove the veil over their hearts.
The Day and Hour
Jesus had repeatedly said that no one knows the day or the hour of His return, and the same thing is true about the time when the “fullness of the Gentiles” will come to pass. It may be that the time is fulfilled when the very last Gentile is saved on the earth, just prior to Jesus’ second coming. We are not told precisely when that last Gentile might be saved before God opens the hearts of the Jews. Either way, it is after the fullness of the Gentiles that “all Israel will be saved, as it is written, “The Deliverer will come from Zion, he will banish ungodliness from Jacob” (Rom 11:26). God will supernaturally open their hearts like “The Lord opened [Lydia’s] heart to pay attention to what was said by Paul” (Acts 16:14). I’ve witnessed to several Jewish friends, but none of them is interested in hearing about Jesus Christ; even though He too was Jewish.
The Bible tells us that the fullness of the Gentiles is coming…and the Lord Jesus Christ is coming again, but we do not know the moment when either of those things will happen. We do know how God feels about Israel, and that “God has not rejected his people whom he foreknew. Do you not know what the Scripture says of Elijah, how he appeals to God against Israel” (Rom 11:2)? Some day in the future, God will once again gather His people unto Himself and bless His people with prosperity and peace, but that time has not yet come. It is coming, but not until the “fullness of the Gentiles” has come.
Here is some related reading for you: What Does it Mean to Harden Your Heart? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.