Jacob Emden, also known as Ya'avetz (June 4, 1697 – April 19, 1776), was a leading German rabbi and talmudist who championed traditional Judaism in the face of the growing influence of the Sabbatean movement.

[4] The acronym Ya'avetz (יעב"ץ, also written Yaavetz) stands for the words Yaakov (Emden) ben Tzvi (his father's name, יעקב (עמדין) בן צבי).

[7] Until the age of seventeen, Emden studied Talmud under his father Tzvi Ashkenazi, a foremost rabbinic authority, first at Altona, then at Amsterdam (1710–1714).

In 1715 Emden married Rachel, daughter of Mordecai ben Naphtali Kohen, rabbi of Ungarisch-Brod, Moravia (the modern Czech Republic), and continued his studies in his father-in-law's yeshivah.

He was soon attacked for his publication of the siddur (prayer book) Ammudei Shamayim, due to his harsh criticisms of the powerful local money changers.

The Yud Ayyin combination of Yaavetz would stand for Yaakov (rather than Yosef, or Yehuda), and the remaining two letters representing ben Tzvi : יעקב בן צבי.

However, there have been allegations that ''Megillat Sefer'' was tampered with, and had deliberately ridiculous accusations, as well as outlandish tales, written in to the original in order to make a mockery of Emden.

[14] The majority of the community, including Aryeh Leib Epstein of Königsberg, favored Eybeschütz; thus the council condemned Emden as a slanderer.

His life seeming to be in actual danger, in May 1751 he left the town and took refuge in Amsterdam, where he had many friends and where he joined the household of his brother-in-law, Aryeh Leib ben Saul, rabbi of the Ashkenazic community.

Conflicting testimony was put forward and the matter remained officially unresolved[16] although the court sentenced the council of the three communities to pay a fine of one hundred thaler for civil unrest and ordered that Emden be allowed to return to Altona.

One Friday evening (July 8, 1755) his house was broken into and his papers seized and turned over to the "Ober-Präsident" (royally imposed mayor), Henning von Qualen (1703–1785) [de].

[20] The recent discovery in Metz of notarial copies of the disputed amulets written by Eyebeschutz support Emden's view that these are Sabbatean writings.

Emden's opinions were often viewed as extremely unconventional from the perspective of strictly traditional mainstream Judaism, though not so unusual in more free-thinking Enlightenment circles.

[23] In 1772 Frederick II, Duke of Mecklenburg-Schwerin, having issued a decree forbidding burial on the day of death, the Jews in his territories approached Emden with the request that he demonstrate from the Talmud that a longer exposure of a corpse would be against the Law.

"[26] Emden praised the ethical teachings of Christianity, considering them beneficial in removing the prevalence of idolatry and bestowing gentiles with a "moral doctrine".

He also wished to revoke the ban on polygamy instituted by Rabbeinu Gershom believing it erroneously followed Christian morals, although he admitted he lacked the power to do so.