Rabbi Kaplan was a prime force behind the teshuvah phenomenon the return to Jewish observance. Throughout history, Jews have always been observant, he once remarked. The teshuvah movement is just a normalization. The Jewish people are sort of getting their act together. Were just doing what were supposed to do.
In culling Jewish sources for his books, he once remarked, I use my physics background to analyze and systematize data, very much as a physicist would deal with physical reality. This ability enabled him to undertake monumental projects, producing close to 50 books, celebrated for their erudition, completeness and clarity.
His personal example of modesty, midos tovos, great human warmth and sensitivity, and total dedication to Torah study and life of mitzvos, was an inspiration to the thousands of individuals whom he touched. His home was always open to visitors, great and humble, from every segment of the Jewish community. His Shabbos table was always crowded with guests attracted to the beauty of the Torah life that he lived, and to the endless stream of wisdom and Torah insight which flowed from his lips.
He labored tirelessly, day and night, producing more outstanding works of great and original Torah scholarship single-handedly than teams of other authors working in the field. Yet, he somehow managed to find time for the simplest Jew, perfect strangers, seeking the answers to the spiritual questions in their lives. None were turned away empty-handed.
Sources: Orthodox Union