At the end of the Book of Ruth, Boaz redeems the Moabitess Ruth, and the people and elders of the city say to Boaz: “We are witnesses. May the Lord make the woman, who is coming into your house, like Rachel and Leah, who together built up the house of Israel. May you act worthily in Ephrathah and be renowned in Bethlehem, and may your house be like the house of Perez, whom Tamar bore to Judah, because of the offspring that the Lord will give you by this young woman” (4.11-12, ESV). Ruth goes on to give birth Obed, the father of Jesse, the father of David. Thus a foreign woman is incorporated into the direct line of the Messiah’s earthly family (see Matt. 1.5-6; Luke 1.31-2).
Ambrose, in his commentary on Luke, thus sees in this marriage a type or foreshadowing of the gathering of all the nations in Christ–that is, of the universal kingdom of the gospel. He writes:
When Boaz, the great-grandfather of David, saw Ruth’s behaviour, her devotion to her mother-in-law, her loyalty to her dead husband, and her fear of God, he chose her for his wife in accordance with the law of Moses which bade him raise up offspring for his next of kin. That this marriage was symbolic is shown by the blessing given by the elders: “May the Lord make this woman who is about to come into your house like Rachel and Leah, who together built up the family of Israel. May she make you powerful in Ephrathah and renowned in Bethlehem. And may your house be like the house of Perez, whom Tamar bore to Judah, through the offspring the Lord gives you by this young woman. And Boaz took Ruth and she became his wife,” and she bore Obed, the father of Jesse and grandfather of David.
Matthew did well, then, when about to summon all nations to the Church through the Gospel, to recall that the Lord who brings about this gathering of the nations was himself, in his human body, of alien origin. Matthew thus made known that it was from this lineage that he would come who was to summon the nations – he whom we desire to follow, we of alien origin who were gathered together when we left our native land and said to whoever called us to worship the Lord, Paul, for example, or any bishop: “Your people shall be my people, your God my God.” So did Ruth, like Leah and Rachel, forget her own people and her father’s house and, freeing herself from the fetters of the law, she entered the Church.
What good reason there was for inserting Ruth’s name in the lineage of the Lord is shown by the revelation of a still more profound mystery, for in the words: “May the Lord give you power in Ephrathah and make your name renowned in Bethlehem” it is prophesied that Christ should be her descendant. For what is this power if not that by which the Christ gathered together all the nations of the world? Whence is this renown if not in the fact that Bethlehem became the Lord’s hometown when he was born as a man. As the prophecy proclaims: “And you, Bethlehem, in the land of Judah, are by no means least among the towns of Judah, for from you shall come the prince who will rule my people Israel.” 1