45 “Again, the kingdom of heaven is like a merchant seeking beautiful pearls,
46 who, when he had found one pearl of great price, went and sold all that he had and bought it.
In the parable of the merchant looking for fine pearls, Christ revealed that God will get a treasure not only from the nation Israel but from the Gentiles as well. This is inferred from the fact that a pearl comes out of the sea. Frequently in Scripture the sea represents Gentile nations. Once again we see that a treasure from among the Gentiles becomes God’s by purchase. — Pentecost, page 218
If this take is correct, and I think it is, the pearl refers to the Gentile nations that will be blessed through Israel during the Millennial Kingdom. It does not refer to the church because 1) the Church wasn’t revealed until the revelation was given to Paul, and 2) the Church won’t be in the kingdom.
“And now the Lord says, who formed Me from the womb to be His Servant, to bring Jacob back to Him, so that Israel is gathered to Him (for I shall be glorious in the eyes of the Lord, and My God shall be My strength), indeed He says, ‘It is too small a thing that You should be My Servant to raise up the tribes of Jacob, and to restore the preserved ones of Israel; I will also give You as a light to the Gentiles, that You should be My salvation to the ends of the earth’” (Isaiah 49:5-6).