The 52nd prohibition is that we are forbidden from marrying heretics.1

The source of this commandment is G‑d's statement,2 "do not intermarry with them," which then explains what kind of intermarriage is referred to — "do not give your daughters to their sons, and do not take their daughters for your sons."

Tractate Avodah Zarah3 states clearly, "the Torah prohibition applies where there is marriage."

There are variations regarding the punishment for one who transgresses this prohibition. If he has relations with her in public, the punishment is carried out by one who kills him while he is in the act — as Pinchas did to Zimri.4 Our Sages said5 [regarding this law]: "a man who has relations with a non-Jew — the zealous are allowed to kill him." This is true only upon certain conditions: that the act was done in public and the act is still in progress, just as it happened [with Pinchas and Zimri].

If, however, nobody knows of the act, or he separated from her before the zealous could kill him, his punishment is kares. This kares is not explicitly mentioned in Scripture. Our Sages asked,6 "what if the zealous do not kill him?" They explain that he receives kares, as it is written,7 "[Yehudah has rebelled...] because Yehudah, who was holy to G‑d, has profaned himself; because he loved and had relations with the daughter of a foreign god. G‑d will give kares to the one who does this, [he will neither be] a student nor a teacher." [In reference to this] our Sages said, "from here we learn that he receives kares."

Whenever it is determined that a man has had relations with a non-Jewish woman, and there were witnesses and a warning,8 he is lashed by Torah law. One should understand this.

The details of this law are explained in Avodah Zarah9 and Sanhedrin.10