[2][3] In 1903, Reuven joined the kollel of Rabbi Chaim Ozer Grodzinsky in Vilnius (Yiddish: Vilna).

Both Rabbis Maskileison and Katz would engage in Torah discussions with students from the nearby Mir Yeshiva.

At the same time, Rabbi Katz was working on his sefer, Degel Reuven, which he would publish in 1922, gaining him further recognition throughout the nation as a serious Torah scholar.

He soon became involved in the Vaad HaYeshivos, the organization dedicated to supporting the yeshivas of Eastern Europe, founded by Rabbi Yisrael Meir Kagan, the "Chafetz Chaim."

It was his activism in the organization that made the Chafetz Chaim choose him to travel to America as their overseas fundraiser in 1929.

[2] In 1932, the Jewish community of Petah Tikvah in Mandatory Palestine offered Rabbi Katz the rabbinate.

Rabbi Katz also wrote a five volume work on the Torah titled Duda'ei Reuven[4] and a sefer titled Shaar Reuven, a collection of essays on contemporary issues, including German reparations, drafting girls and yeshiva students into the army, and celebrating Israeli independence day.