In the third month of 1275, about one year before this letter was written, Nichiren Daishonin warned Shijō Kingo, his loyal samurai follower who was an early convert, that as a practitioner of the Lotus Sutra he must be prepared to meet further difficulties and hardships.

In the present work, the Daishonin explains the nature of true happiness. It lies, he says, in chanting Nam-myoho-renge-kyo. Suffering is unavoidable, he adds, encouraging Shijō Kingo to “regard both suffering and joy as facts of life, and continue chanting p.682Nam-myoho-renge-kyo, no matter what happens.” The Daishonin emphasizes that this is the way to experience “the boundless joy of the Law,” or the state of Buddhahood.

https://www.nichirenlibrary.org/en/wnd-1/Content/86