When we read, “No foreigner is to eat of it. Any slave you have bought may eat of it after you have circumcised him” (Ex 12:43–45), we can understand that those who got circumcised could keep the Passover even though they were Gentiles. And as it is written, “An alien living among you who wants to celebrate the LORD’s Passover must have all the males in his household circumcised; then he may take part like one born in the land” (Ex 12:48), those who get circumcised with faith in God could become God’s people even though they were Gentiles.
In the book of Isaiah, it is written that those who keep the Sabbath and hold fast to God’s covenant with love for God will be led to the holy mountain of God even though they are Gentiles. This clearly shows that even the Gentiles could believe in God.
Around the 6th century B.C., the Jews could not go to Jerusalem as they were held as captives to Babylon. So they built a synagogue in each place and had gatherings there. It seems that the Jews preached the gospel to the Gentiles around this time.
The Ethiopian eunuch who visited Jerusalem to hold worship in the times of the early Church, and Cornelius the Centurion who received the gospel of the New Covenant through Peter were all Judaist who believed in God though they were Gentiles. And on the day of the Pentecost, Gentiles from various countries of the world came to Peter, who was filled with Holy Spirit, and listened to his preaching.
These days too, there are people who believe in Judaism though they are Gentiles. However, as we know, they are only a few.
In the New Testament times, however, the gospel was preached to the Gentiles incredibly. In other words, the gospel of God’s salvation got transferred from the physical Jews to the spiritual Jews.
The prophets recorded in the Old Testament that many Gentiles would come before God to learn the gospel of the New Covenant.
Isa 2:2 In the last days the mountain of the LORD’s temple will be established as chief among the mountains; it will be raised above the hills, and all nations will stream to it.
Mic 4:1–2 In the last days the mountain of the LORD’s temple will be established as chief among the mountains; it will be raised above the hills, and peoples will stream to it. Many nations will come and say, “Come, let us go up to the mountain of the LORD, to the house of the God of Jacob. He will teach us his ways, so that we may walk in his paths.”
God even laid the foundation for preaching the gospel to the Gentiles: He amended the law with regard to food, which was a stumbling block to the Gentiles. In the Old Testament times, God did not allow the Jews to eat unclean food in order to differentiate the Jews from the Gentiles (Lev 11:1–11). But in the New Testament times, they just need to abstain from food sacrificed to idols, blood, the meat of strangled animals, and sexual immorality (Ac 15:28–29).
In the days of the early Church, God chose Apostle Paul to preach the gospel to the Gentiles. When Paul visited Antioch to preach the gospel, the Jews in Antioch mocked and rejected the gospel whereas the Gentiles accepted it in great delight. Then Apostle Paul said, “We had to speak the word of God to you first. Since you reject it and do not consider yourselves worthy of eternal life, we now turn to the Gentiles. For this is what the Lord has commanded us: ‘I have made you a light for the Gentiles, that you may bring salvation to the ends of the earth’ ” (Ac 13:46–47).
Paul and his companions, who arrived at Antioch after making a detour around various places due to hindrances from the Jews, gathered the members together, and reported all that God had done through them and how He had opened the door of faith to the Gentiles, and he suggested that they join it (Ac 14:27).
It is extensively recorded throughout the New Testament about how the gospel of the New Covenant spread to the Gentiles, so it is hardly necessary to reemphasize it.
In conclusion, in the Old Testament times too, the Gentiles were entitled to believe in the gospel, but only a few of them believed. So we can say that the gospel was closed to the Gentiles in the Old Testament times when comparing it with the New Testament times. Only in the New Testament times, the door of the gospel opened wide, and it became possible to preach the gospel to the ends of the earth.