427. Wedding Bell Blues: The prohibition against intermarriage
Do not intermarry with them… (Deuteronomy 7:3)
We are likewise commanded not to make marriages with other nations, neither to give our daughters to them nor take their daughters for us. This mitzvah was stated in the context of the seven Canaanite nations but it does not apply exclusively to them. Due to certain redundancies in the verse, it applies to other nations as well, but with one crucial difference: the Canaanite nations were not permitted to join Israel through conversion, while other nations could convert and then marry in.
The reason for this mitzvah is apparent: we don't want the pagan influence on our homes. We don't want their religious icons in our houses and we certainly don't want them indoctrinating our children. If you want to raise your children in your faith, you have to marry someone who will partner with you in the attempt.
This mitzvah applies in all times and places. It is discussed in the Talmud in the tractates of Yevamos (23a), Sanhedrin (82a) and Avodah Zarah (36b). It is codified in the Shulchan Aruch in Even Ha’Ezer 16. This mitzvah is #52 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #19 of the 194 negative mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.