This letter was written to Toki Jōnin in the seventh year of Kenchō (1255), two years after Nichiren Daishonin established his teaching of Nam-myoho-renge-kyo. At the time of this letter, the Daishonin was thirty-four years old and was living in Kamakura, the seat of the military government. Toki was a staunch follower of the Daishonin who lived in Wakamiya in Shimōsa Province. He received some thirty letters, including Letter from Sado and one of the major treatises, The Object of Devotion for Observing the Mind. A retainer of Lord Chiba, the constable of Shimōsa, Toki had become a follower of the Daishonin around 1254.

https://www.nichirenlibrary.org/en/wnd-1/Content/1