In Chullin 13b, we read:
Rabbi Yoḥanan says: Gentiles outside of Eretz Yisrael are not idol worshippers. Rather, they are just following the customs of their ancestors that were transmitted to them... Rav Yosef bar Minyumi says that Rav Naḥman says: There are no heretics among the nations of the world.
What does this mean? Aren't all idolaters, in Eretz or elsewhere, just following the customs of their society? We don't know what people really believe -- we just observe how they act. So who is a real idolater?
Note: Different from similar question, because I didn't ask whether it was permitted, but why the Talmud seems to say it doesn't even count as idolatry.