The International Standard Bible Encyclopedia, Revised
Intermarriage[Heb hithpael of ḥāṯan—‘become each other’s son-in-law’] (“intermarry,” Ezr. 9:14); AV JOIN IN AFFINITY; NEB JOIN IN MARRIAGE. Intermarriage of the returned exiles with the surrounding pagans would lead Israel back into the idolatry and abomination for which God’s judgment had fallen
Key passages
Ex 34:16

and you take from their daughters for your sons, and their daughters prostitute themselves after their gods, and they cause your sons to prostitute themselves after their gods.

Dt 7:3–4

And you shall not intermarry with them; you shall not give your daughter to their son; and you shall not take his daughter for your son. For their sons and daughters will cause your son to turn away from following me, and so they will serve other gods, and the anger of Yahweh would be kindled …

Jos 23:12–13

for if indeed you turn back and join these remaining nations among you, and you intermarry with them, marrying their women and they yours, know for certain that Yahweh your God will not continue to drive out these nations from before you; they will be for you a snare and a trap, a …

Jdg 3:5–8

And the Israelites lived in the midst of the Canaanites, the Hittites, the Amorites, the Perizzites, the Hivites, and the Jebusites. And they took their daughters as wives for themselves, and they gave their daughters to their sons, and they served their gods. The Israelites …

See also
Topics & Themes