We’ve said regarding the meisis (one who tries to convert other Jews to idolatry) that we are not to legitimize him nor are we to soften our position regarding him. Our current mitzvah is going to be surprising, though. If we see the meisis in trouble – even mortal peril – we are not to help him. Just like the previous tells us that the meisis is an exception to making amends with our enemies, this mitzvah tells us that the meisis is an exception to Mitzvah #237, not standing idly by when another person is in danger.
The reason that some might find this mitzvah surprising is that we all know Judaism to be a compassionate and caring religion. How could it command us to stand by and do nothing when a meisis is in danger? Really, there’s a time to exercise the trait of compassion and a time to be harsh. The Midrash in Yalkut Shimoni (and elsewhere) cites a familiar dictum that one who is merciful when he should act harshly will ultimately act harshly when he should be merciful. The poster child for such errors in judgment is King Saul, who let King Agag of Amalek live, but who killed the kohanim of the city of Nov because he imagined that they had aided David against him. Even in modern times, we know of genocidal maniacs who are good to their dogs. Misplaced compassion is a real danger that the Torah warns against through this mitzvah.
The reason for the mitzvah is, of course, to stop the spread of idolatry. If you save the meisis, he’ll continue with his work, further undermining God in society.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Sanhedrin on pages 29a, 33b, 67a and 85b. It is codified the Mishneh Torah in the fifth chapter of Hilchos Avodas Kochavim. This mitzvah is #19 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #26 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.