Ruth the Moabitess married an Israelite named Mahlon who had been living in Moab. When Mahlon died, Ruth chose to return with her mother-in-law to Israel and become an Israelite herself. She declared to her Jewish mother-in-law, “Your people will be my people and your God will be my God” (Ruth 1:16).

Ruth’s words inspired the Sages and became the formal statement of covenant that has since been uttered by millions of other converts to Israel. Ruth is depicted in Jewish tradition as an ideal wife. The Proverbs 31 ‏אשׁת־חיל (wife of noble character) may be an allusion to Ruth the Moabitess (Prov 31:10; cf. Ruth 3:11) and her values are recounted every Erev Shabbat (Sabbath eve) by Jewish men to their wives. 

Ruth arrived in the Land of Israel and soon after married Boaz, the son of Rahab and Salmon (an intermarried couple) according to the Apostolic Writings (Matt 1:5; cf. Josh 6:25). It is notable that Ruth continued to be called רות המואביה (Ruth the Moabitess) even after becoming a convert to Israel (Ruth 1:22; 2:2, 6, 21; 4:5, 10). This is clearly due to the recognition that converts did not sever themselves completely from their own family or cultural upbringing. They continued to have a tie to their nation of origin. 

Boaz and Ruth had a son named Obed. Obed had a son named Yishai (Jesse). And Yishai had a son named David who became the king of Israel. David’s great grandparents, then, were an intermarried couple, as were his great great grandparents. The book of Ruth, which is read by Jews every year during the harvest festival of Shavuot, memorializes the life story of Boaz and Ruth. It is an eternal reminder that even a marriage between a Jew and a Moabite can be a marriage made in heaven. 

King David had several wives. One of them was a non-Israelite by the name of Maacah. We do not know much about her except that she was the daughter of Talmai, king of Geshur, a pagan ruler (1 Chron 3:2). Through this intermarriage, Absalom and Tamar were born (2 Sam 13:1). Absalom killed his stepbrother Amnon for raping Tamar and fled to his pagan grandfather's kingdom where he lived in exile for several years (2 Sam 13:37). Absalom eventually returned to Jerusalem and led a conspiracy against his father that resulted in David fleeing the palace with Maacah. Absalom sought to kill his father but ended up being killed instead (2 Sam 14-18). King David also had a sister named Abigail who married an Ishmaelite named Jether (1 Chron 2:15-17). Thus, King David's father (Jesse) had two children who intermarried.

Rabbi Arthur Blecher reminds us in his recent book The New American Judaism that Jews have been intermarrying for thousands of years and that the Torah, the central document of Jewish law, does not contain a universal prohibition of intermarriage:

Intermarriage is as old as the Jewish people. Of the 613 commandments in the Torah, the first five books of Scripture that form the core of Jewish law, no commandment categorically forbids a Jew to marry a gentile. In fact, in Exodus 2:21, the Hebrew Bible recounts that Moses married the daughter of a Midianite priest. Even though she was of a different religion, their son, Gershon, was without question one of the Children of Israel. The Torah completely accepts the legitimacy of Moses’ relationship with this non-Israelite woman and takes for granted the Israelites status of their offspring. A later passage in the Torah (Numbers 12:1) tells that Moses married an Ethiopian woman. Elsewhere (Ruth 4:10-17) the Bible describes the marriage between the Moabite woman Ruth and the Israelite man Boaz as an ordinary fact of daily life. The descendent of that mixed marriage, David, became the king of Israel (pp. 164-165).

There is an intermarriage prohibition in the Bible. However, this prohibition is not universal – it prohibits Jews from intermarrying with people from the seven nations of Canaan (Deut 7:1-4; Exod 34:15-16). Since there were other peoples with whom the Israelites came into contact – Egyptians, Edomites, Moabites, Midianites, etc. – one must assume that intermarriage was not forbidden with these neighboring peoples or the more distant nations. Shaye Cohen, Professor of Hebrew Literature and Philosophy at Harvard University, concurs in his book The Beginnings of Jewishness:

Does this prohibition apply to all gentiles or only to the seven Canaanite nations? The answer is clearly the latter. Moses commands the Israelites to destroy the seven Canaanite nations because they threaten Israelite religious identity and live on the land that the Israelites will conquer. Intermarriage with them is prohibited. The Ammonites and Moabites, somewhat more distant and therefore somewhat less dangerous, were not consigned to destruction and isolation; they were merely prohibited from entering the congregation (Deut. 23:4). The Egyptians and Edomites were even permitted to enter the congregation after three generations (Deut. 23:8-9). The meaning of the prohibition of “entering the congregation” is not at all clear, as I shall discuss below, but I presume that originally, at least, it was not a prohibition of intermarriage. Other nations, even further removed from the Israelite horizon, were presumably not subject to any prohibition. Internal biblical evidence confirms this narrow interpretation of Deut. 7:3-4 (pp. 242-43).

Though there is not a universal commandment in the Torah prohibiting intermarriage, there is an underlying assumption throughout the Torah that a Jew should marry someone (from within the nation or without) who affirms the covenant God made with Israel. Both spouses in an intermarriage should be committed to upholding the covenant responsibilities incumbent upon members of the nation of Israel, which includes raising children to express their love for God by living out these responsibilities. 

Intermarriage often leads to assimilation but it does not have to. Sometimes the Gentile Christian spouse is more committed to keeping a Jewish home and raising Jewish children than the Jewish spouse.