"An Amonite or a Moabite shall not enter the congregation of Hashem." (Devarim 23:4)
In the Book of Ruth we read that an aristocratic family of Jews - Elimelech, his wife Naomi and their two sons Machlon and Kilyon - left the Holy Land during a famine and sojourned in the Land of Moav. While there, they married the Moabite princesses Ruth and Orpah. Subsequently Elimelech, Machlon and Kilyon died, and Naomi decided to return to her people. Ruth and Orpah wanted to come along with her, but she attempted to dissuade them, saying that she was too old to bear any more sons for them to marry, and even if she did, it would not be worth waiting for them. She encouraged them to go back to Moab, get married and live non-Jewish lives. Orpah went back, but Ruth stayed with Naomi, proclaiming, "Your people is my people, and your G-d is my G-d."
We must ask: when exactly did Ruth convert to Judaism? It does not make sense that a great man like Elimelech would let his sons marry non-Jewish women, so they must have converted before marrying them. But then why would Naomi encourage them to go home? Once someone has a converted to Judaism, she can never convert back. So it would seem that Ruth's conversion took place on the road back to the Holy Land. But then why do we find that Boaz said, "When you buy the field from Naomi, you will have acquired Ruth the Moabitess, the wife of the deceased, to raise up the name of the deceased upon his estate." Why was there a need to fulfill the concept of yibum - marriage to a relative of the deceased husband - if Ruth was never really married to Machlon?
The Midrash Rabbah (Ruth 2:9) takes the view that Ruth and Orpah did not convert at the beginning: "They did not convert them, nor immerse them, and the law had not yet been passed that only an Amonite or Moabite man is forbidden, not a woman." According to the Yefei Anaf commentary (by Rabbi Shmuel ben Yitzchak Yaffeh Ashkenazi), the Midrash means that at that time, the Torah's law against intermarriage with the Moabites was thought to apply to Moabite women as well as men. However, the law only applies to a Moabite convert to Judaism who legally marries a Jew. A Jew who has an extramarital relationship with a non-Jewish Moabite woman does not violate any Torah law, as long as it takes place in private. Machlon and Kilyon therefore made sure that their wives did not convert, and their relationships were considered private, since there were no other Jews around.
But then we have our question: why did Boaz have to marry Ruth and "raise up the name of the deceased husband" if she was never really the deceased's husband?
Therefore, I would like to answer this based on the Rambam (Isurei Biah 13:14-17), who addresses the question of how Shimshon and Shlomo Hamelech could have married gentile wives. He answers that in those days, the official Jewish courts did not accept any converts, because they suspected all converts of converting for ulterior motives - to share in the glory and power the Jewish people had at that time. But unofficial courts (any group of three Jewish men is considered a court) accepted converts, and their Jewishness was considered doubtful - until they proved themselves sincere. Shimshon and Shlomo certainly converted their wives in this manner. Since Shimshon's wives ended up more faithful to the Philistines than to the Jews, and Shlomo's wives ended up worshipping their old idols, Scripture calls them "foreign wives," but legally speaking they were still Jews. This is because they proved themselves sincere prior to changing their minds and reverting to idol worship.
Here too, we can say that Ruth and Orpah converted for the sake of marriage, and their true status was in doubt for many years. When their husbands died and Orpah returned to idol worship, this proved that she had never converted to begin with. But Ruth sincerely wished to be a Jewess, and thus her conversion and marriage to Machlon was valid all along. This is why she had to marry a relative of her husband.
Later, I found that the Zohar Chadash (Rus 180-182) gives a similar explanation.
We can explain that this is also what the Midrash means: "They did not convert them nor immerse them" means that the official Jewish court did not convert them. Aside from the suspicion that they were converting only for marriage, the Midrash says that there was an additional problem that "the law had not yet been passed that Moabite women are permitted." This was the true law, as given to Moses at Sinai, but most Jewish scholars had forgotten it. It was only Elimelech's family that kept the tradition alive. Therefore, Machlon and Kilyon, who knew that women were permitted, certainly convened an unofficial court to convert Ruth and Orpah. But as stated above, their conversions were still dependent on their true intent. In the end, only Ruth proved to be sincere.
The fact that this law was not well-known until Boaz’s time is stated in the Midrash (Ruth Rabbah 4:1). The Midrash quotes a verse in Divrei Hayamim I (8:8), “And Shacharayim fathered a child in the field of Moab, from Shilcho, Chushim and Bara his wives. And he fathered through Chodesh his wife…” Shacharayim was Boaz, who was free (meshuchrar) from sin. He was descended from Yehuda, who was called Shilcho (because he was sent ahead into Egypt). He hurried (chush) like a leopard to clarify (bi’er) the law that only the men of Amon and Moab are forbidden, not the women. He fathered from Chodesh (=new) his wife – because in his days the law was rediscovered (nischadshah) that the women of Moab are permitted.
In another place the Midrash (Ruth Rabbah 2:4) tells us that the entire episode of Ruth was part of the hidden Divine plan. Divrei Hayamim I 4:22 reads: "And Yokim and the men of Kozeva and Yoash and Saraf, who married into Moab, and Yashuvi Lachem - and the matters are hidden."
The Midrash expounds: Yokim is Elimelech. The men of Kozeva are his sons, who expired from the world (mechuzavim). Yoash - they gave up on Eretz Yisroel. Saraf - they burned the Torah. Rabbi Menachem said in the name of Rabbi Acha: Did they really burn it? This teaches that whoever fails to keep one thing in the Torah is considered as if he burned it. Who married into Moab - they married Moabite wives, they left the Jewish people and joined the country of Moab. And Yashuvi Lechem - this is Naomi, who returned with Ruth from Moab to Beis Lechem. And the matters are hidden - this has already been explained by two Sages, each according to his way. Looking earlier in the Midrash, we find that it is explained: "These things were said by the Hider of the World - G-d, who orchestrates the world in hidden ways." Since Ruth was to lead to the birth of King David and eventually moshiach, G-d had to hide this episode. The conversion of Ruth could not take place in an official court, and so G-d caused the law permitting her to be forgotten from the general court system for many years. Only Elimelech and his family knew of it. Her conversion had to be handled privately and secretly.
The Midrash also contains a hint to this idea, expounded upon in the Zohar Chadash, that Ruth converted twice. Ruth 2:12 reads: "May Hashem repay your action, and may your reward be complete from Hashem, G-d of Israel, under whose wings you have come to take shelter." The Midrash reads: "May Hashem repay your action, and may your reward be complete from Hashem - Rabbi Chasa said: Under whose wings you have come to take shelter." Why did Rabbi Chasa have to say words that were written explicitly in the verse? Some say this is a scribal error, but it could be that he came to say that Ruth actually converted twice - once when she married Machlon, and once when she chose to accompany Naomi back to Eretz Yisroel, thus proving her previous conversion valid.