If I haven't convinced you of this thread running throughout the entire Old Testament, Warren Wiersbe's summary of 8:20-23 should:
The Gentiles will be redeemed (Zech. 8:20–23). God called Abraham and established the nation of Israel so His people would witness to the Gentiles and lead them to faith in the true God (Gen. 12:1–3). In setting apart one nation, God was seeking to reach a whole world. Many of the great events in Jewish history recorded in Scripture had behind them a witness to “the whole world”: the plagues of Egypt (Ex. 9:16); the conquest of Canaan (Josh. 4:23–24); God’s blessing of the nation (Deut. 28:9–11); and even the building of the temple (1 Kings 8:42–43). When David killed Goliath, he announced that God would give him victory so “that all the earth may know that there is a God in Israel” (1 Sam. 17:46).But Israel failed in her mission to the Gentiles. Instead of the Gentile nations coming to worship the true God of Israel, the Jews forsook Jehovah and worshiped the false gods of the Gentile nations. The “court of the Gentiles” in Herod’s temple became a market where Jews visiting Jerusalem from other countries could exchange their money and buy approved sacrifices. However, before we criticize the Jews too much, we had better examine the track record of the church when it comes to winning the lost at home and taking the Gospel to nations abroad.When Messiah restores His people and establishes His kingdom, the Gentiles will trust the true and living God and come to Jerusalem to worship Him. Isaiah saw a river of Gentiles “flowing” into the city (Isa. 2:1–5) and Micah used the same figure (Micah 4:1–5). Zechariah describes a scene in which ten men (a Hebrew expression for “many men”) will take hold of one Jew and beg to go with him to the temple!It’s a wonderful thing when God so blesses His people that others want what God’s people have. “We have heard that God is with you” (Zech. 8:23). This sounds like what should happen in our local churches when an unbeliever beholds our worship of the Lord. “He will be convinced by all that he is a sinner and will be judged by all, and the secrets of his heart will be laid bare. So he will fall down and worship God, exclaiming, ‘God is really among you!” (1 Cor. 14:24–25, niv)“I say then,” wrote Paul, “has God cast away His people? Certainly not!” (Rom. 11:1, nkjv) There’s a bright and blessed future for God’s people Israel, even though the nation has been oppressed and persecuted by the Gentiles, some of whom claimed to be Christians. Our privilege is to love them, pray for them, and tell them that their Messiah, Jesus Christ, has come and will save them if they trust in Him. The Gospel of Christ is still “the power of God for salvation to everyone who believes, for the Jew first and also for the Greek” (Rom. 1:16, nkjv).

