“Then they will know that I am the Lord their God because I made them go into exile among the nations, and then I gathered them again to their own land; and I will leave none of them there any longer. I will not hide My face from them any longer, for I will have poured out My Spirit on the house of Israel,” declares the Lord God. – Ezekiel 39:28-29

Many biblical prophecies have both an immediate and future fulfillment. After having been exiled to Babylon for 70 years, the nation of Judah (under the reign of the Persian king Cyrus) returned to Jerusalem. This event was an immediate fulfillment of Ezekiel’s prophecy and it occurred hundreds of years before the time of Christ and the arrival of the Holy Spirit at Pentecost.
Pentecost may also be considered a fulfillment of Ezekiel’s prophecy, but it too is only partial. The ten northern tribes of Israel were exiled and scattered by the Assyrians long before the Judeans were dragged away by Babylon. They have not been returned en masse (even though some might say that they are returning now) implying that there must be a future (perhaps spiritual) fulfillment to come.
Be assured that whatever God has said will happen, will happen. His Word never fails.
When God promises, He’s not saying, I’ll try. He means, I can, and I will. – James MacDonald