In Psalm 66:1-4, we find these wonderful words: "Shout for joy to God, all the earth; sing the glory of His name; give to Him glorious praise! Say to God, 'How awesome are Your deeds! So great is Your power that Your enemies come cringing to You. All the earth worships You and sings praises to You; they sing praises to Your name.'" The anonymous Psalmist makes a joyful call to the nations of the world to praise the God of Israel, the only true and living God. Then he makes a call equally joyous to God to take the Gentiles into His favor.
Let's consider, first, what it is not saying. Classical dispensationalism taught that the church was unknown in the Old Testament. Folks holding that system of doctrine claim that Jesus intended to create a political kingdom at His first coming, but was prevented by the unbelief of the Jews. As a result, He was forced to turn to the Gentiles, to build an unplanned church, until He returns to his plan for Israel during the millennium.
However, in this Psalm, we see that God had had a plan for bringing the Gentiles into His church long before Jesus's earthly ministry, and His Old Testament people knew about that plan (even if they lost sight of it by New Testament times).
It is prophecies such as this one that are the basis of Paul's joyous hope for the Gentiles. That prospect was so valuable to him that he named himself the Apostle to the Gentiles (Romans 11:13)! He understood the Hebrew prophecies to describe a turning of the Gentiles to the Jewish God, as those who "who were not My people" and "not loved" were instead to be called "sons of the living God" (Hosea 1:6, 8, 10; Romans 9:24-27). This is not "replacement theology" (as meaningless as that phrase is). While the Gentiles have benefited from the hardening of Israel, "Through their trespass salvation has come to the Gentiles, so as to make Israel jealous" (Romans 11:11), and there is greater benefit yet to come, "if their trespass means riches for the world, and if their failure means
riches for the Gentiles, how much more will their full inclusion mean?" (verse 12), the benefit will also pass the other way, "a partial hardening has come upon Israel, until the fullness of the Gentiles has come in. And in this way all Israel will be saved" (verses 25-26). When the number of Gentiles is complete, then ethnic Israel will again be revived, and the joining of the two groups will be so joyous as to be as if they had risen from the dead!