…do not cover up for him. (Deuteronomy 13:9)
If the meisis tried to talk others into joining him in idolatry, what about his victims? Those whom he tried to tempt should be permitted to speak out against their tempter at his trial. They are encouraged to do so and we are enjoined to let them. This, however, is a negative commandment: we are not to refrain from speaking against the meisis.
As with the past several mitzvos, the reason here is that the meisis, who promotes idolatry, has set himself as an opponent of God. As members of God’s team, it goes against our interests to assist him. (See Psalms 138:21 – “Don’t I hate those who despise You, Hashem?”)
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 #21 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #24 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim; it is not listed among the mitzvos that can be observed today in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.