You shall not bear affection for him… (Deuteronomy 13:9)
Just as we may not legitimize an idolatrous “prophet” by asking for more information, we may not encourage a “meisis.” A meisis is someone who tries to persuade other Jews to worship an idol by telling them how great it is. We are not to engage the meisis in discussion or befriend him in any way as doing so only perpetuates his idolatrous missionizing.
The reason to shun the meisis is the same as for the idolatrous prophet: our job as Jews is to shut down those who are trying to recruit for idols, not to give them a foot in the door.
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 #17 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.