Rabbi Mordechai Dov Altein, a long-serving Chabad-Lubavitch rabbi who was instrumental in the establishment of yeshivahs and other Jewish educational institutions in post-war America, passed away on Sunday. He was 100 years old.

Born on the Lower East Side of Manhattan in 1919, Altein would later recall that life in New York during that time was not an easy one, especially for an observant Jew. His father was a peddler, buying and selling various odds and ends. As his father was self-employed, he was able to keep Shabbat, which was virtually impossible for almost anyone who worked for a business, or in a factory or a shop.

In the 1920s, Altein attended Yeshivah Chaim Berlin, one of the only yeshivahs in New York, with an enrollment 700 to 800 students; in the 1930s, he attended high school at Yeshivah Torah Vodaath in the Williamsburg neighborhood of Brooklyn, N.Y. While a high school student, he met Rabbi Yisrael Jacobson, who was sent by the Sixth Rebbe, Rabbi Yosef Yitzchak Schneersohn, of righteous memory, in the 1920s to strengthen Judaism in America. Altein and some high school friends began studying with Jacobson, taking classes in the Tanya, Rabbi Schneur Zalman of Liadi’s seminal work of Chabad Chassidic philosophy, and various other Chassidic texts.

“It opened our eyes and was like a breath of fresh air for us,” Altein once told an interviewer. “One can’t imagine how difficult it was to remain a religious Jew at the time. Everyone was trying to assimilate so as to gain entry into American culture. Very few sent their children to yeshivot—perhaps one out of every hundred children went to learn in a Jewish school. It was very hard to fight the tide, to have the courage to stand out and be different. Most of my friends from school ended up attending secular high schools and universities. Thank G‑d, though, what we learned gave us the courage to keep fighting, and remain religious. Without Chassidut, we wouldn’t have made it.”

Jacobson encouraged the teens to correspond with the Sixth Rebbe, who suggested that the young men travel to Poland to learn in the main Chabad Yeshivah in Otwock.

“Keep in mind that this was the summer of 1939, the eve of Second World War,” said Altein. “But the Rebbe told us to come, so we went. When we first arrived, I was allowed to pray in the Rebbe’s minyan that Shabbat. The regular yeshivah students had their own prayer services in the yeshivah, so joining the Rebbe was a very special honor.”

After returning to America, Altein continued his studies at Lubavitcher yeshivahs. On 17 Kislev 5702 (1941), Altein, then a 22-year-old student, was called into the office of the Sixth Rebbe, who had arrived in the U.S. a year earlier, for a private audience. Altein was told to pack his bags immediately and go to Pittsburgh to take up a post in a synagogue. Just days before the 19th of Kislev, the Rosh Hashanah for Chassidic study, Altein wanted to stay behind and hear the Rebbe deliver the annual discourse given on that day, a momentous occasion for Chassidim. But the mission required that he pack his bags and leave, and he was off the next morning to Pittsburgh.

In 1943, Altein married Rochel Jacobson, the daughter of his teacher. The Rebbe—Rabbi Menachem M. Schneerson, of righteous memory—and Rebbetzin Chaya Mushka Schneerson, of righteous memory, attended the couple’s engagement celebration, and the Rebbe was the mesader kidushin, officiating at their wedding. It was a marriage that would last for 76 years.

Following the wedding, the Sixth Rebbe sent Altein to establish yeshivahs in Pittsburgh; New Haven, Conn.; and the Bronx. The Alteins remained in their Bronx home for decades, long after most in the Jewish community decamped for other neighborhoods.

“The [Seventh] Rebbe refused to let my grandfather leave if there were Jewish residents in the immediate area,” Rabbi Yisrael Pinson, director of Chabad of Greater Downtown Detroit, told Chabad.org. It was only when there was another Chabad House in the Bronx, in Riverdale, that the Alteins considered moving. “Central to my grandfather's world view was his dedication to what the Rebbe said,” noted Pinson.

In addition to his role as an educator, Altein also served for many years as a member of the Vaad Raboinei Lubavitch and other leading rabbinical organizations. He also took great pride in providing the Rebbe with a lulav and etrog set each year. The practice began when his father-in-law, Rabbi Jacobson, sourced an etrog from Calabria for the Sixth Rebbe during a visit in 1929. Beginning in the 1950s Altein continued the practice by traveling to Italy each year, where he would hand-select etrogim for the Rebbe. The first year he supplied a lulav and etrog to the Rebbe, the Rebbe paid with a check, but Altein wouldn't cash it, so the Rebbe switched to paying with cash. Altein would keep the money as well, often giving the bills the Rebbe paid with to his grandchildren to keep.

Rabbi Mordechai Dov Altein is survived by his wife, Rebbetzin Rochel, and their children: Chanie Gurarie (Montreal, Canada); Rabbi Avrohom Altein (Winnipeg, Canada); Rabbi Leibel Altein (Brooklyn, N.Y.); Malka Cohen (Manchester, England); Sara Pinson (Nice, France); and Sima Zalmanov (Tzfat, Israel) in addition to many grandchildren and great-grandchildren. He was predeceased by his son, Yossi,obm. He is also survived by his sister, Miriam Popack, of the Crown Heights neighborhood of Brooklyn, N.Y.

The levaya will take place Monday, leaving Shomerei Hadas at 1:30 p.m. and passing by 770 Eastern Parkway at 2:30 pm. Shiva will be at 1378 Union Street. Visiting hours are 10 a.m to 1:30 p.m; 3:30 p.m. to 9:00 p.m. Condolences can be sent to nichumaltein@gmail.com.