Nichiren Daishonin wrote this letter in the tenth month of the eighth year of Bun’ei (1271), just before he left for his exile on Sado Island. At that time, he was being held at the mainland residence of Homma Rokurō Saemon, the deputy constable of Sado. The Daishonin wrote the letter to an acquaintance at Seichō-ji temple in Awa Province, possibly a priest named Enjō-bō.

Exile to Sado Island was a harsh punishment, second only to the death penalty. In this letter the Daishonin declares that he has met this persecution solely for the sake of the Lotus Sutra; he emphasizes that the very fact that it has happened demonstrates that he is “reading” the Lotus Sutra with his life. Since the Daishonin seeks to dispel doubts among his followers and revive their flagging courage by pointing out his mission as the votary of the Lotus Sutra in the Latter Day, this letter may be said to foreshadow The Opening of the Eyes, a major treatise he wrote four months later.

https://www.nichirenlibrary.org/en/wnd-1/Content/25