This letter is actually a response to one sent to R’ Salanter from his students whom he’d been writing to all along. It seems that they’d said in their letter to him that they’d faced strong opposition at home to the Mussar movement that R’ Salanter had abjured them to set in place, and that R’ Salanter was now responding to that. R’ Salanter himself experienced a lot of criticism from opponents and had to draw upon the truth of the need for Mussar study and its great worth to go forward, and that he was now asking his students to do the same.
As such, he told them not to “fear the voice of the mighty storm” of opposition, and that they weren’t to “become anxious” about their prospects for the future. Indeed, they were to encourage those around them to delve into Mussar study, since (among other things) it protects those who are “drowning in the forces of their habitual (untoward) longings”.
After all, he underscored, “the study of Mussar calls out, ‘Life’!” to those who’d listen to it. That is, Mussar encourages one to live fully cognizant of his stellar capabilities as a human being, and to delve into that his whole life long. And R’ Salanter pointed out that in fact the “effort required” for Mussar study and mastery “is small, while the rewards are great”.
And he wisely offered that if, despite his efforts, a student of Mussar nonetheless feels “that he hasn’t awakened, and he feels that no impression (has been made) upon his soul” that he shouldn’t despair, because that will happen in time and he’ll indeed be elevated.
In fact, that’s how the great Rabbi Akiva grew — step by step. We’re taught in Avos d’Rebbe Nossan that Rabbi Akiva was forty years old and hadn’t yet studied Torah, and he wondered how he’d ever manage to succeed by that point in his life. Here’s the epiphany that helped.
We’re taught that “once while standing next to a well he wondered (out loud), ‘Who chiseled this stone?'” He was told that it was the waters that dripped upon it day by day that chiselled it. So he said, “if what is soft (the water) can carve what is hard (the stone), then surely words of Torah which are as hard as iron will (eventually) penetrate my heart, which is flesh and blood”, which happened in fact.
R’ Salanter’s point is that that can be your experience too if you slowly but steadily study Mussar texts. (The subtle, unstated implication here is that the slow encouragement of a Mussar Movement will also eventually succeed in piercing the hearts of its opponents.)