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.