Halakhah for Shabbat 133:13

To not follow the practices of the gentiles: To not follow the practices of the Amorites and so [too,] the practices of the gentiles, as it is stated (Leviticus 20:23), "And you shall not follow the practice of the nation that I am driving out before you." And the law is the same for all the nations, since the matter is that they turn away from [following] God, and worship idolatry. And the content of the commandment is that we not behave like them in our clothing and our matters. And it is like they said in Sifra, Achrei Mot, Chapter 13:8, "'And do not follow their practices - that you not follow their mores with things that are fixed for them, such as theaters, circuses and amphitheaters" - and all of these are types of frivolity that they would do in their gatherings, when they gathered to do craziness, licentiousness and idolatry. And they said there, "'The practice of the nation' - Rabbi Meir says, 'These are the ways of the Amorites that the sages numbered (see Shabbat 67a).' Rabbi Yehudah ben Betira says, 'That you should not grow a tassel of the head and not cut its growth'" - meaning to say that he not shave from the sides and leave hair in the middle, which is called a forelock. And this negative commandment is repeated in another place with other words, as it is stated (Deuteronomy 12:30), "Guard yourself lest you be ensnared to follow them." And the language of Sifrei is "'Guard' is with a negative commandment; 'lest' is with a negative commandment; 'you be ensnared to follow them' is lest you imitate them and do like their deeds; 'and it shall be a snare for you' is that you not say, 'Since they go out with velvet, I will go out with velvet, since they go with helmets, I will go with a helmet'" - and that is a type of knight's armor. And the language of the books of prophecy (Zephaniah 1:8) is "and upon all of the dressed, there is a foreign dress."

The laws of the commandment are in scattered places in the Talmud, but mainly in the seventh chapter of Sanhedrin. And anyone who is a judge about it needs to know the wisdom of magic so that he will be able to distinguish about an act that is done, whether it is one of the types of magic or perhaps from the things done through the power of nature and in permissible ways. And [it is] like the matter that they, may their memory be blessed, said (Shabbat 67a), "Anything that contains [an element] of healing does not contain [the prohibition] on account of the ways of the Amorite." And I have already spoken according to my ability about this, there on the negative commandment of "Do not keep alive a witch." And these matters require great analysis - as behold, acts are found in the Gemara that if we did not know them from their mouth, may their memory be blessed, we would have forbidden them from a concern about this prohibition. But behold nonetheless, one who raises his soul to enter into these machinations - and compares one thing to another from those things that they, may their memory be blessed, mentioned [as being permissible] - is like one who opens an opening to go to Gehinnom.