The Fast day of the 9th of Teves:
The Shulchan Aruch lists a number of days in which it is customary to fast due to tragedies that occurred on that day. Amongst the list of days in which tragedies occurred, and is hence proper to fast on, is the 9th of Teves. [Practically, it is no longer customary to fast on these days, and thus we no longer fast on the 9th of Teves. Nonetheless, below we will explore the background behind this fast and its meaning.]
What happened on this day? We do not know what tragedy occurred on this day. [Some Poskim, however, write that in truth Ezra Hasofer passed away on this day, as written in the Selichos of Asara Biteves. In addition, some say that also Nechemia Ben Chakilia passed away on this day. They thus question the above assertion that the tragedy of the day is unknown. The Poskim, however, explain that although Ezra Hasofer did pass away on the 9th of Teves, an additional tragedy also occurred on this day for which the fast was established, and it is this tragedy that is left unknown. We will now explore what secret tragedy could the Sages have been referring to.]
What secret tragedy occurred? While the above asserted tragedy was kept hidden, as stated above, we do find some revelation amongst Sefarim as to what occurred, and the reason it was kept secret is blatantly obvious, to shield the Jewish people from the wrath of the Christians, as we will now see. Some Poskim write that the secret tragedy that happened on this day is that on this day Oso Ish [Yoshka] was born. Other Poskim write that on this day Shimon Kipa passed away. Shimon Kippa, also known as Shimon Hakalfus, was none other than S. Peter, who was the first Bishop/Pope of Rome of which all the other popes are considered as his heirs. This Shimon Kippa who became the founder of Christian theology after the passing of Yoshka, was in truth a hidden Tzadik, who helped sway Christianity away from Judaism and make it unattractive to Jews. He also helped save the Jewish people from persecution from the christians, and hence the Sages established this day, which is the day of his death, as a fast day for the scrupulous, in commemoration of his passing of the. [See below for greater background of Shimon Kipa/Peter and how they were Tzaddikim and acted as emissaries of the Sages to infiltrate Christianity.]
Who were Paul and Peter of the Christian religion?
According to Chazal, Paul [Yochanon] and Peter [Shimon Kipa] 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. Paul and Peter were entrusted by the Sages to infiltrate the religion as a student of Yoshkeh and after his passing to divert the religion from any similarity to Judaism, and remove it from targeting any Jews. They performed miracles using the names of Hashem and thus gained the trust of the followers of Yoshkeh, as being the new spokesman of Yoshkeh, after his death. They successfully dissected christianity from its Jewish roots, changed Shabbos to Sunday, nullified the 613 commands, and preached to the followers not to harm the Jews. They wrote the books of the Christians of luke, mathew, mark, john and others. Peter, Shimon Kipa, eventually had them build him a cave in a mountain in Rome, under the claim that he needed a secluded area to speak with Yoshkeh, when in truth he studied Torah there, kept Kosher, and sent letters to the Sages professing his righteousness. These letters included Piyutim, liturgy, which were later entered into the liturgy of Jewish prayer of all the Jewish people.
In the words of Rashi:
Rashi states as follows: “The Gemara states that all the writings and language of the nations is not from them. This means as follows: All of their books of heresy Yochanon Paulus [Paul] and Petrus, who were Jews, wrote. They purposely infected their culture in order to sway the Christian faith away from Judaism. 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 Judaism. Peter was appointed by the Sages to do so.
Author of Nishmas:
There is a tradition recorded that Peter [Shimon Kipa] authored Nishmas and certain Piyutim which are said on Yom Kippur by all Jewry, in the Seder Avoda of Musaf. Other sources however negate the tradition that Peter wrote Nishmas.
 Michaber 580:1-2; Tur 580; Kol Bo 63; Bahag [900’s; times of Geonim] Hilchos Tishe Beav Vetaanis 39; M”A 580:1 in name of the Tanya, in name of the Geonim, and Shibulei Haleket; Megillas Taanis [written by the Tanaaim], brought in M”A ibid and Gr”a; Shlah p. 201, brought in Kaf Hachaim 580:2
Who established these fasts? See M”A 580:1 in name of the Tanya, in name of the Geonim, and Shibulei Haleket that we do not know who established these fast days and so writes Beis Yosef 580. However, the M”A ibid concludes that he found them written in Megillas Taanis, hence proving that it was established in the times of the Tanaaim
 Beis Yosef 580 “I have never seen anyone fast on these days and I have also not heard of anyone doing so”;
Background and other opinions: The Bahag and Tur ibid write these fasts are Biblical, however the Beis Yosef 580:1 explains that this is not to be taken literally, and rather it means to say that it is Rabbinical but should be treated as if it were Biblical. The Beis Yosef concludes that “I have never seen anyone fast on these days and I have also not heard of anyone doing so. In truth these fasts are a great wonderment, and I wonder at who established them, as some were established even on Rosh Chodesh”; The Michaber 580:1 simply records that it is proper to fast on this day and does not write it as an obligation; The M”A 580:1 writes in name of the Tanya, in name of the Geonim, and Shibulei Haleket that we do not know who established these fast days. Nonetheless, the M”A concludes that since he has read the lists of these fasts in Megillas Taanis, and also the Bahag [of whose words are all Divrei Kabalah], therefore it must have been written in the times of the Tanaaim and every Baal Nefesh is to be stringent to fast on these days if he is able. Practically, we no longer fast on these days
 Michaber 580:2 “On the 9th of Teves we do not know what Tzara happened”; Tur 580 and Bahag ibid “The Sages did not write what happened”; Orchos Chaim “On the 9th the Sages did not write what happened, and it remains a secret.”; Birkeiy Yosef
 Taz 580:1; M”A 580:6; Perisha 580:6; Beir Hagoleh 580 and Elya Raba 580:5 in name of Kol Bo; Bahag ibid, brought in P”M 580 M”Z 1; Orchos Chaim
Passing of Rav Yosef Hanaggid: In the Sefer Kabbalah of the Raavad it is written that on the 9th of Teves Rav Yosef Halevi Hannagid was killed, the son of Rav Shmuel Hanaggid. Together with him the entire congregation of Graneda and many other Jews were killed. [See Hagahos Baruch Frankel 580]
 So is written in the first Piyut of Selichos of Asara Biteves “זועמתי בתשעה בו [בחודש טבת] בכלמה וחפר, חשך מעלי מעיל הוד וצפר טרוף טורף בו הנותן אמרי שפר, הוא עזרא הסופר”
 Bahag ibid, brought in P”M ibid; Orchos Chaim
 Taz ibid concludes “Vetzaruch Iyun Rav”
 Birkeiy Yosef 580 based on Rishonim brought next; See Kol Bo and Bahag ibid who writes “The Sages did not tell us what happened, and [in addition it happens to be that] on that day died Ezra Hakohen and Nechemia Ben Chakilia”; See also Orchos Chaim “On the 9th the Sages did not write what happened, and it remains a secret, and on this day died Ezra Hakohen and Nechemia Ben Chakilia” The wording of all these Rishonim imply that the death of Ezra and Nechemia is in addition to the main reason of establishment, and the Sages did not write what actually happened and it is left a mystery.
 Haghos of Tosafos Yisheinim in name of Gadol Echad, brought in the Hosafos of some editions of the Shulchan Aruch on chapter 580 [Machon Yerushalayim-Friedman edition] removed from regular editions due to censorship of the church
 Hagahos Baruch Frankel on 580 “I found in a manuscript that on the 9th of Teves Shimon Hakalfus, who helped save the Jewish people from a great tragedy in the times of the Peritzim, died and the Sages established it as a day of fasting in Jerusalem”; See Sefer Toldos Yeshu “Shimon Hakalfus passed away and the Jewish people mourned his passing and they established his day of death as an annual fast day. This day was the 9th of Teves”; Accordingly, the words of Toldos Yeshu and Baruch Frankel reveal the secret that on this day Shimon Hakalfus died.
 See Sefer Chassidim 191 [Mosad Rav Kook-censored from some versions] “Even a Tzaddik who they make into a deity it is a Mitzvah to call a derogatory name, for example Shimon Kipah should be called Peter Chamur” thus implying that that Shimon Kipa, who was Peter, was a Tzaddik; Rashi Avoda Zara 10a which was censored and brought in Dikdukei Sofrim Avoda Zara p. 12 footnote 9 that they were Tzadikim planted in the Christian religion; For the full story in all details see: Sefer Toldos Yeshu [dating back to at least 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]
 Rashi Avoda Zara 10a which was censored and brought in Dikdukei Sofrim Avoda Zara p. 12 footnote 9; For the full story in all details see: Sefer Toldos Yeshu [dating back to at least 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]
 See 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 Moshiach comes” Vetzaruch Iyun, as elsewhere in Machzor Vitri it states that Shimon Kipa authored the Piyutim of Seder Avoda said on Yom Kippur.