Not to mention the name of idols and foreign deities:
Giving it a derogatory name: One is required to help eradicate idolatry and give idols and foreign deities a derogatory name.
Swearing using its name: One who swears in the name of an idol or foreign deity [transgresses a negative Biblical command and] is liable for lashes.
Mentioning the name of an idol: It is forbidden to mention the name of an idol or foreign deity. This applies whether there is a need to mention his name, such as to tell his friend to wait for him by a certain idol, and whether there is no need to do so, and one wants to mention his name in casual conversation. It is permitted to mention the names of idols written in the Torah, such as Kara, Baal, Koreis, Nevo, Haorchim Lagad Shulchan, [Baal Tzafon]. [This however only applies to names of idols and deities that were innovated for the sake of the idolatry, and hence the meaning of the name itself connotes a foreign god [such as Zeus, which connotes Deity in Greek]. However, common names of people/items which have been used for idols, may be mentioned, as the name does not have any godly connotation, and was not innovated for this purpose. Thus, we find that the Gemara mentions the names of festivals of idolatry, as well as the name of Oso Ish and his students.]
Mentioning their festivals: It is permitted to mention the names of their festivals that are called the same names as people, although one may not refer to it in a manner of eminence as do their worshipers. [It is forbidden to mention the name of a festival that is called the name of their idol or deity.]
It is forbidden to mention the name of an idol even for location purpsoes, if the name is unique to idols, connotes a deity, and is not used for the names of people. It is permitted to mention names of idols that were originally used as names of people/items, and hence do not innately connote a deity. The same applies for the names of their Holidays, it may be mentioned, although without reverence, if their names are also common names of people. It is permitted to mention the name of any idol for the sake of poking fun at it.
May one write the names of idolatry and false deities?
It is permitted to write the names for learning purposes.
May one say the names of idols that are now extinct, such as the false deities of Greek mythology, such as Zeus/Cronus/Hera/Hades?
Seemingly, it is permitted to mention names of idols that have become extinct and are no longer worshipped. [Nonetheless, one must make sure that in truth this idol is no longer revered or served. There exist today modern paganists, which although not popular still worship some of the above Greek deities, such as Zeus.]
May one say the name Jesus or Yeshu?
From the letter of the law it is permitted to recite the name Jesus or Yeshu. It is likewise permitted to write these names, as we find Gedolei Yisrael who wrote these names in their Sefarim. Nevertheless, despite the letter of the law, the custom of all Jewry dating back many generations is to avoid saying these names and rather the term “Oso Ish” or “Yoshka” or “Yoshke Pandre” is used in its stead. One is not Heaven forfend to break this custom.
May one say the word Christ?
One is not to use this term as it connotes a Messiah and savior, and according to some even a deity, and so is the custom of all Jewry to not say this term.
May one say the name Chris-mass?
No, and so is the custom. One is rather to use a epithet [i.e. nickname] such as Kratzmacht; Nittle, and the like. Seemingly however the term X-mass is not to be used, as the X is short for Ch***, and is used also by Christians as a formal name of the holiday.
May one say the name Mary?
From the letter of the law it is permitted to do so, although G-d fearing Jews avoid saying this name [when in reference to the mother of Yoshka].
May one say the names of Saint Paul/Peter/Patrick/Francis?
From the letter of the law, one may say their names without mentioning their status of sainthood. Thus, while one may say Paul/Patrick in reference to the apostles, one may not say a preface to his name which connotes reverence, such as Saint Paul. One is also not to write such a preface of reverance, and possibly should not even write its initial, such as S. Peter, because people may come to read it in full. [It however may be written for learning purposes, as stated earlier in the Q&A.] Furthermore, it is proper to mention their names in a derogatory form when possible, such as Peter should be called Peter Chamur. [According to Chazal, Paul and Peter were Tzaddikim who were planted into the christian world in order to divert their religion away from resemblance to Judaism, and they remained faithful to the Jewish people throughout their lives. Nevertheless, they are still to be called by a derogatory name.]
May one say the name of a city that is named after an idol or Saint?
From the letter of the law, it is permitted to do so if the city is named after a saint. It is likewise permitted to say the name if the city is named after an idol whose name does not connote a deity and is used as a name of people or items, as stated above. Nevertheless, G-d fearing Jews are accustomed not to say the name of such cities but rather give it a nickname. For example, the city of Bela Tzurkav in Russia they would call Shevartza Tzurkav. If the city name is of a false deity whose name connotes a deity in the language originated, and is not used as a name of people or items, then [from the letter of the law] it may not be used to call the name of the city. [An example of such a city is perhaps Mumbaiy, which is the name of a pagan goddess which connotes the term “Mother of creation.” One should call it Bombay or other name of the like.ll it Bombay or otehr Mumbaiy, which is a pagan godesseity in the langugae er gave it a derogatory term.g therd d ]
Saying the word Saint of the city name: If the name is preceded by the term Saint, such as S. Diego, S. Paul, S. Monica, and the like, then it is not to be said or written even in initials, as stated in the previous Halacha. [The city of S. Monica is commonly called Simcha Monica by Chassidim. Nevertheless, when writing the name of a city in Gittin, one is to write the name as required by Halacha, even if it involves writing the preface of S.]
 Michaber 146/15; Avoda Zara 47b
 Michaber 147/1; Mishneh Sanhedrin 60b
 Michaber 147/1; Sanhedrin 63b
 This is learned from the verse “Vesheim Elokim Acheirim Lo Sazkiru, Lo Yishama Al Picha”
 Michaber ibid; See Taz 147/1 for the novelty of this ruling;
Other opinions: Some Poskim rule that when there is an absolute necessity one may mention the name. [Chavos Yair Teshuvah 1 Hasaga 11-12]
 Shach 147/2; Perisha
 Michaber 147/4; Sanhedrin 63b
The reason: As these idols have become extinct and nullified. Alternatively, since the Torah mentions it, then certainly we may mention it. [Yireim 245 ; See Tofas Riem for differnet Girsaso and if these are two reasons or one; Levush 147/4; See also Chavos Yair Teshuvah 1 Hasaga 11-12 who questions the first reason; See Likkutei Sichos 23/166 that the Torah’s mentioning of the name destroys it and makes the idol worthless]
 Yireim 245  “If they were given a name that connotes a deity”, brought in Hagahos Maimanis Avodas Kochavim 5/3; Chavos Yair Teshuvah 1 Hasaga 11-12; Biur Hagr”a 147/3; Teshuvos Rav Azriel Hildsheimer 180
The reason: As only those names that were innovated for the sake of idolatry was the Torah particular against one mentioning. [Poskim ibid] See Chavos Yair ibid that according to Yireim ibid even names that were innovated for a deity may be used if they do not connote a meaning of a deity in the language. However Chavos Yair questions this and asserts that perhaps only names that were already used for other purposes have this allowance.
 Poskim ibid; See Sanhedrin 43; 67; 105; 107; Avoda Zara 27; Yerushalmi Brachos 5/1
 Michaber 147/2; Mordechai in name of Ravayah; Hagahos Maimanis; Rabbeinu Yerucham
 Michaber 147/5; Rav Nachman in Sanhedrin 63b
 See Sefer Chassidim 427 that only to a Ger should one not mention the name of an idol even to make jest of it.
 Chavos Yair Teshuvah 1 Hasaga 11-12; Teshuvos Rav Ezriel Hildsheimer 180
 This is simialr to the allowance to mention names of idols written in the Torah, of which the Yireim 75 writes that the reason is because they have become extinct/nullified. Vetzaruch Iyun as not all Poskim agree with this reason.
 The reason: As it is permitted to recite names of people who do not connote a deity and were later turned into a deity. [Yireim brought in Hagahos Maimanis Avodas Kochavim 5/3; Chavos Yair Teshuvah 1 Hasaga 11-12; Biur Hagr”a 147/3; Teshuvos Rav Ezriel Hildsheimer 180; See Mishneh Halachos 9/169]
 See Biur Hagr”a ibid; This name is mentioned in various places in the Gemara and Rambam; See Sanhedrin 43; 67; 105; 107; Avoda Zara 27; Yerushalmi Brachos 5/1; However see Teshuvos Rav Ezriel Hildsheimer 180 that no proof can be brought from here that these names may be recited as there is no prohibition to write the names, and rather the prohibition is simply to say them.
 Teshuvos Rav Ezriel Hildsheimer 180; See Mishneh Halachos 9/169
 Teshuvos Rav Ezriel Hildsheimer 180; See Mishneh Halachos 9/169
 Although we find Sefarim that write this term as well, nevertheless one cannot learn from them that it is permitted to say the names, as writing is more lenient than saying, as well as that it is permitted to write the names for learning purposes. [ibid]
 Chavos Yair Teshuvah 1 Hasaga 11-12
 The reason: In addition to all the reasons of allowance mentioned above regarding Yoshka, this woman is not worshiped or considered a G-d by even the Christians, and hence it has no relation to idolatry. [ibid]
 Beis Yosef 147/2 in name of Rabbeinu Yerucham “However their Kedoshim/saints may be called by their names if they are names of people, although to call them by their names in a manner of reverence is forbidden.”; See also Michaber 147/2; Kneses Hagedola 147/4, brought in Darkei Teshuvah 147/7; See Likkutei Sichos 26/429 [printed in Shulchan Menachem 4/34-35] “Mentioning their names, even with a preface does not border the prohibition of “Lo Sazkiru”, as their name is not a name of Avoda Zara.” Nontheless, a preface should not be mentioned.
 Likkutei Sichos 26/429 [printed in Shulchan Menachem 4/34-35] that one may not say the term Saint, and therefore there is a question if one should even write the initial S., let alone the actual word, lest people come to say it.
 Likkutei Sichos ibid that Rebbe questioned whether the letter S. may be written before the name, even if it is the name of a city, such as S. Petersburg, and the Rebbe reffered the asker [who was a Tanya printer in such a city] to a Rav. Practically, the name published in that Tanya does not contain the S. [See Shulchan menachem ibid footnote 1]
 Sefer Chassidim 191 “Even a Tzadik who they make into a deity it is a Mitzvah to call a derogatory name, for example Shiimon Kipah should be called Peter Chamur”
 See Sefer Chassidim ibid that Peter was a Tzadik; See Rashi Avoda Zara 10a which was censored and brought in Dikdukei Sofrim Avoda Zara p. 12 footnote 9 “The Gemara states that all the writings and language of the nations is not from them. This means as follwos: All of their books of heresy Yochanon Paulus [Paul] and Petrus which were Jews wrote. They purpsoesly inbfected their culture in order to sway the Christian faith away from judaisim. They themselves were not heretics and did so for the benefit of the Jewish people, as written in the book Teliya Yeshu” Seemingly this refers to the book Toldos Yeshu, which brings the history of Yeshu from a Jewish perspective and writes how Peter infiltrated the ranks of Yeshu and turned away Christianity from being a threat to Judaisim. Peter was appointed by the Sages to do so. For the full story in all details see: Sefer Toldos Yeshu [dating back to at leats times of Rashi]; Beis hamidrash vol. 5/60 and vol. 6 [1860; Likkut of old Midrashim] Midrash brought in Otzer Hamidrashim p. 557 [1920, by Rav Y.D. Eizanshtein]
Fast day of 9th of Teves: See Hagahos Baruch Frankel and Toldos Yeshu “On the 9th of Teves Shimon Hakalfus who helped save the Jewish people died and the Sages established it as a day of fasting”; See also Michaber 580/2 “On the 9th of Teves we do not know what Tzara happened”; See Taz 580/1 and M”A 580/6 who say Ezra died and question Michaber. However see Tur, Bahag; Orchos Chaim, Birkeiy Yosef who write that the Sages did not write what happened and it is left a mystery, and the fact Ezra died is not the reason for the fast. Accordingly, the words of Toldos Yeshu and Baruch Frankel reveal the secret that on this day Shimon Hakalfus died. Shimon Hakalfus was none other than Shimon Kifa, whose Christian name was S. Peter. He was the first Bishop/Pope of Rome and all the other popes are considered his inherators.
Author of Nishmas: See also Siddur Avodas Yisrael, in name of an old manuscript siddur from the year 1407 “I heard from Yehuda Bar Yaakov that Shimon Ben Kipa authored Nishmas until the words Mi Yidmeh Lach” However see Machzor Vitri p. 282 Mahadurah Makitzei Nirdamim “Some say Nishmas was authored by the abomination of Rome, called Shimon Peter Chamor, which authored this liturgy and others while in the cave. Vechas Veshalom to say such a thing, and one who says so will have to bring a fat offering when Moshaich comes” Vetzaruch Iyun, as elsewhere in Machzor Vitri it states that Shimon Kipa authored the Piyutim opf Seder Avoda said on Yom Kippur.
 Sefer Chassidim ibid reagridng Shimon Kipa who was a Tzadik that he should be called Peter Chamur
 The reason: As it is permitted to say names of idols that are also names of people, as brought from Poskim ibid, and the same would apply if these names are also names of cities. This allowance certainly applies if the name is merely that of a worshiper of idolatry and not an actual idol. [See Likkutei Sichos 26/429 [printed in Shulchan Menachem 4/34-35]
 See Likkutei Sichos 26/429 [printed in Shulchan Menachem 4/34-35]
 See Likkutei Sichos ibid that Rebbe questioned whether the letter S. may be written before the name, even if it is the name of a city, such as S. Petersburg, and the Rebbe reffered the asker [who was a Tanya printer in such a city] to a Rav. Practically, the name published in that Tanya does not contain the S. [See Shulchan menachem ibid footnote 1]
 See Likkutei Sichos 26/429 [printed in Shulchan Menachem 4/34-35]; See Shulchan Menachem ibid footnote 5 regaridng the wording of the city name in a Get written in Bela Tzurkav, that they used this name even though the Jews avoided calling it by this name, and rather gave it a derogatory term.