First, allow me to list some Bible references for this: Genesis 2:24, 31:19; Deuteronomy 7:1-4; Ruth 1:16-18; Esther 1:10-22; Ezra 9:1-2, 10:1-14.
In the Ancient Near East (ANE), there was a particular recurring problem that was addressed by the Covenant of Moses and in Persian Law. In the ANE, most royal and noble marriages were diplomatic unions, where marriage was a means of establishing peaceful relations between the ruling dynasties of vassals and allies. These women often brought their own private staff and some property, and typically had their own quarters where they conducted affairs in her home language, different from her husband’s language. As time wore on, this semi-independence became much more pronounced.
This became a particular problem in the Book of Esther where Queen Vashti in her own quarters had set up a parallel domain of her own, with her own politics and so forth. She dared to defy her husband, the emperor, when ordered to appear for a grand imperial feast. The imperial decree he eventually issued in essence made this common practice illegal because of the precedent it set. A woman must leave her world and became fully integrated into her husband’s world, lest her continued separation from it fracture the marriage and the whole empire.
It was the same issue in Israel’s covenant, but with a nuance we typically miss in our English translations: the “foreign” wives were so defined because they were not joining the Covenant. There was nothing wrong with marrying a foreign woman who converted and joined the nation by coming under the Covenant (like Ruth). It was when the woman in question retained her own deities that it was unlawful. This is what kind of women Ezra ordered the Returnees to divorce. In essence, these were not legitimate marriage covenants; the women were classed as concubines.
Thus, when Solomon collected a massive harem, some were wives who converted, but too many were concubines who remained idolatrous. Solomon’s practice seemed worldly wisdom, but destroyed his faith. This was a threat to the Covenant and the blessings of shalom.
Granted, Paul wrote a lot about how to deal with complications of people coming to Christ after already being married. This shows the growing pains of trying to build a covenant community of faith from scratch in a heathen society, but it’s just about the same with a secular society. Worst of all, we are faced with a Western brand of “Christianity” that remains deeply mired in a heathen culture while pretending otherwise.
We need to commit now to raising up a new covenant society, with an eye to the multi-generational effects of doing it right from the start. This is a massive undertaking, even if the society stays small, because it means a broad-based resistance to the prevailing secular culture in defining a lifestyle that is self-consistent and heart-led.