In 1851 Hildesheimer was appointed rabbi of the Austro-Hungarian community of Eisenstadt; there he reorganized the educational system and established a yeshivah, where secular studies were included in the curriculum. The yeshivah was highly successful, and students came there from all over Europe. However, the great majority of Orthodox Hungarian rabbis bitterly opposed his modernism and the institution he created. In 1869 Hildesheimer accepted a call from Berlin to become rabbi of the newly founded Orthodox congregation, Adass Jisroel. In 1873 he established a rabbinical seminary which later became the central institution for the training of Orthodox rabbis in Europe. Hildesheimer shared with S. R. Hirsch the leadership of the Orthodox Jewish community of Germany. He was an active worker on behalf of stricken Jewish communities throughout the world. Throughout his life, he was an enthusiastic supporter of Palestine Jewry and the building of the yishuv. The Battei Mahaseh dwellings in the Old City of Jerusalem were erected on his initiative. In 1872 he founded a Palaestina Verein with the object of raising the educational and vocational standards of Jerusalem Jews, particularly by the establishment in 1879 of an orphanage. This drew on his head the bitter antagonism of the ultra-Orthodox old yishuv, which placed him under a ban (herem). Hildesheimer supported the Hovevei Zion and the colonization movement.

The geonim were recognized by the Jews as the highest authority of instruction from the end of the sixth century or somewhat later to the middle of the 11th. In the 10th and 11th centuries the title of gaon was also used by the heads of academies in Erez Israel. In the 12th and 13th centuries - after the Gaonic period in the exact sense of the term - the title gaon was also used by the heads of academies in Baghdad, Damascus, and Egypt. Apparently, the term gaon was shortened from rosh yeshivat ge'on Ya'akov (cf. "the pride of Jacob," Ps. 47:4). Other explanations of the origin of the term offered by modern scholars are not acceptable.

Added t.p.: Responsen der Lehrer des Ostens und Westens, nach Handschriften herausgegeben und erklaert von Dr. Joel Mueller... ראינו טופס, שבו מדבקה על שם ההוצאה והדפוס ועליה: Verlag von M. Poppelauer נדפס תחילה בהמשכים ב"בית תלמוד", ד-ה, תרמ"ד-תרמ"ה.