Nichiren Daishonin wrote this letter on the fifth day of the tenth month, 1271, only three weeks after he was nearly executed at Tatsunokuchi. It was sent to three of his leading disciples: Ōta Saemon, a government official, the lay priest Soya Kyōshin, and the Dharma Bridge Kanabara. One of them may have visited the Daishonin while he was being held in detention for exile at the residence of Homma, deputy constable of Sado Island, in Echi. Records indicate that the three disciples lived in Shimōsa Province, to the northeast of Kamakura; this letter may well have been an expression of gratitude for the visit and for their concern for the Daishonin’s safety.

Following the failure to behead the Daishonin, the government had difficulty deciding what to do with him, so he was temporarily detained at Homma’s residence. Just at that time, a wave of arson and murder swept Kamakura, and the Daishonin’s followers were blamed. The government then directed that the exile that had been ordered earlier be carried out.

The community of believers in Kamakura was deeply upset by this series of events, and the Daishonin sent a succession of letters to reassure them. In this letter, the Daishonin says that hardships allow one to purge oneself of accumulated evil karma in order to bring forth the state of Buddhahood.

https://www.nichirenlibrary.org/en/wnd-1/Content/24