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.
Fast day of 9th of Teves:
The 9th of Teves was established as a fast day for the scrupulous, in commemoration of the passing of the hidden Tzadik, Shimon Kipa, on that day.
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.
 See Sefer Chassidim ibid 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 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 inheritors.
 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.