His wide-ranging literary output, inclusive of introductory pamphlets on Jewish beliefs, and philosophy written at the request of NCSY are often regarded as significant factors in the growth of the baal teshuva movement.

On his first day at the minyan, Henoch Rosenberg, a 14-year-old Klausenburger Hosid, realized that Len was out of place—he was not wearing tefillin or opening a siddur—and befriended him.

[11] After his time in Los Angeles, Kaplan had a few small jobs including teaching at a Hebrew school in the Bronx and at Beth Torah in Richmond, Virginia (February 1955).

That year, he received semikhah (rabbinic ordination) from some of Israel's foremost poskim, including Yitzhak HaLevi Herzog and Eliezer Yehuda Finkel.

He would converse with teenagers and answer their questions, whether in his home or at drawn-out NCSY conventions where "Aryeh Kaplan was the last adult standing.

Pinchas Stolper's wrote in his introduction to The Aryeh Kaplan Anthology how he "discovered" Kaplan:[2] I first encountered this extraordinary individual when by chance I spotted his article on "Immortality in the Soul" in "Intercom," the journal of the Association of Orthodox Jewish Scientists, and was taken by his unusual ability to explain a difficult topic - one usually reserved for advanced scholars, a topic almost untouched previously in English - with such simplicity that it could be understood by any intelligent reader.

I always counted as one of my greatest z'chusim (a spiritual merit granted by God) to have had the privilege of "discovering" Rabbi Kaplan.

The book - masterful, comprehensive, inspiring yet simple - set a pattern which was to characterize all of his succeeding works.Kaplan became involved with Breslov through Rabbi Zvi Aryeh Rosenfeld.

"Using a manual typewriter, Kaplan put together a presentation there on the spot with maps of Ukraine to show exact longitude and latitude".

Rabbi Teitz then sent the presentation to Robert Lipshutz and less than two weeks later after Jimmy Carter met with Leonid Brezhnev at the Strategic Arms Limitation Conference in Vienna, the Soviet ambassador to the U.S. stated that "the Kremlin has decided to honor the plan as originally scheduled, except for Bilinsky Street.

"[44] From 1976 onward, Kaplan worked to translate Me'am Lo'ez (Torah Anthology), which was originally written in Ladino and in time edited for Hebrew (1967).

He put forward creative and original ideas and hypotheses, all the time anchoring them in classical works of rabbinic literature.

[46] His works have been translated into Czech, French, Hungarian, Modern Hebrew, Portuguese, Russian, German and Spanish.