Who was Yohanan ben Zakkai?

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Answer:

Rabbi Yohanan ben Zakkai, disciple of Hillel, found favor in the eyes of the Roman leader Vespasian, for a reason explained in the Talmud (Gittin 56a-b), and was granted a request. He requested that the Romans (who had already begun the process of the Second Destruction) spare the town of Yavneh and its Torah-scholars. Vespasion acquiesced (Talmud, ibid), and Rabbi Yohanan gathered whatever Torah-sages he could, to Yavneh.

After the Destruction, it was these sages (Rabbi Eliezer, Rabbi Yehoshua, Rabbi Akiva and many others) who redeemed Jewish captives, distributed charity, and rebuilt whatever they could.

In matters of Torah, they recorded the laws of the Temple so that these wouldn't be forgotten, and redacted the details of such Mishna-tractates as Shekalim, Yoma, and Tamid. They finalized the text of the Haggadah shel Pesach (Passover Seder), composed prayers for the rebuilding of the Temple, and set in motion the process of redacting the entire Mishna, an effort which would be completed a century later.

Through all these actions, they gave the Jewish people new hope, showing them that the nation could continue even under the new circumstances, with its Torah and traditions (Oral Law) clarified and accessible.