Negative commandment · Lo Ta’aseh #52 of 613
Not to marry gentiles
שלא להתחתן בעובדי עבודה זרה
The verse · Devarim 7:3
וְלֹא תִתְחַתֵּן בָּם בִּתְּךָ לֹא־תִתֵּן לִבְנוֹ וּבִתּוֹ לֹא־תִקַּח לִבְנֶךָ
“neither shalt thou make marriages with them; thy daughter thou shalt not give to his son, nor shalt thou take his daughter to thy son.”
The Rambam’s words
Not to marry gentiles, as [Deuteronomy 7:3] states: "Do not marry among them."
שלא להתחתן בעובדי עבודה זרה, שנאמר "ולא תתחתן, בם" (דברים ז,ג).
Why · Sefer HaChinuch #427
To not marry with idolaters
“It is from the roots of the commandment [that it is that] since most of the masses — in their stupidity — are drawn after the counsel of their wives, if one marries the “daughter of a foreign god,” she will draw him to worship idolatry.”
Who & when
“And it is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה