Nichiren Daishonin wrote this letter at Minobu in the seventh month of the third year of Kōan (1280) to the lay nun Sennichi, who lived on Sado Island. Sennichi was the wife of Abutsu-bō Nittoku, who had passed away the year before.
Abutsu-bō had originally been a Nembutsu believer, but soon after encountering the Daishonin, together with his wife, became his sincere follower and helped provide him with food and supplies during his stay on Sado.
After the Daishonin was pardoned from his exile and had taken up his residence at Minobu, Abutsu-bō, despite his advanced age, made at least three journeys to see him. He died on the twenty-first day of the third month, 1279, at the age of ninety-one. His son, Tōkurō Moritsuna, visited the Daishonin at Minobu later that year with his father’s ashes and laid them to rest there.
The lay nun Sennichi was concerned about the privations of the Daishonin’s life at Minobu, and in 1280 sent Tōkurō with various offerings for him. Tōkurō arrived at Minobu on the first day of the seventh month, and Nichiren Daishonin wrote this letter to the lay nun in acknowledgment of her sincere offerings, entrusting it to her son on his return journey.
In the beginning portion, the Daishonin declares that to read even one phrase of the Lotus Sutra is equivalent to reading all the teachings expounded by Shakyamuni Buddha during his lifetime. He assures the lay nun Sennichi that all persons who embrace the Lotus Sutra will attain Buddhahood, and therefore, in the light of the “clear mirror of the Lotus Sutra,” there can be no doubt that her late husband, Abutsu-bō, has attained Buddhahood as well.
In the remaining portion of the letter, the Daishonin encourages the lay nun in the face of her loneliness following her husband’s death, and also praises the filial devotion of her son, Tōkurō. Especially touched by Tōkurō’s two visits to Minobu for his father’s sake, the Daishonin concludes his letter by exclaiming, “Surely, there is no treasure greater than a child, no treasure greater than a child!”
https://www.nichirenlibrary.org/en/wnd-1/Content/149