This letter was written to Hyōe no Sakan Munenaga, the younger of the two Ikegami brothers. Though it was originally thought to have been written in the first year of Kenji (1275), it is now apparent that it was written in 1277. Munenaga is thought to have taken faith in the Daishonin’s teaching around 1256, shortly after his elder brother Munenaka. Both were officials in the Kamakura shogunate, and their father, Yasumitsu, was director of the government’s Office of Construction and Repairs.

Yasumitsu was a loyal follower of the priest Ryōkan of the True Word Precepts school who was highly active in political affairs. He strenuously opposed their beliefs and disowned Munenaka twice, in 1275 and again in 1277. By disowning Munenaka, Yasumitsu in effect was provoking a rift between the two sons, tempting the weaker Munenaga to trade his beliefs for the right to inherit his father’s estate. Supported by the Daishonin’s guidance and encouragement, however, Munenaga upheld his faith together with his brother, and in 1278, after a total of twenty-two years’ practice, their united efforts finally led their father to accept faith in the Daishonin’s teaching.

https://www.nichirenlibrary.org/en/wnd-1/Content/77