The Rebbe rebukes his doubt, stressing that helping Jews remain faithful is a sacred mission, even if it interrupts study. Citing Chazal, he explains that when one teaches others, his own Torah gains greater light. This “bitul Torah” is itself fulfillment, refining mind and heart, bringing true success in learning and avodah.
https://www.torahrecordings.com/rebbe/004_igros_kodesh/elul/1198