This letter was written at Minobu to Nanjō Shichirō Jirō, commonly known as Nanjō Tokimitsu, in the second month, 1282, when Nichiren Daishonin himself was seriously ill.

When he was in his teens, Tokimitsu had assumed his deceased father’s duties as steward of the Ueno district, which covered a vast area on one side of Mount Fuji. Particularly during the Atsuhara Persecution, Tokimitsu had made many sacrifices in order to defend the Daishonin’s followers who lived in his domains. For his courage, the Daishonin had honored him by naming him “Ueno the Worthy” in a letter written on the sixth day of the eleventh month, 1279, and entitled The Dragon Gate.

On first hearing of Tokimitsu’s grave illness, the Daishonin had apparently asked a disciple to write a letter of encouragement on his behalf since he himself was too ill to write. Deeply concerned, however, about the youthful believer, he forced himself to take up his writing brush and sent this letter through Nikkō to help Tokimitsu overcome his illness.

The Daishonin declares that Tokimitsu is a person who, according to the Lotus Sutra, has made offerings to a hundred thousand million Buddhas in his past existences. He then strictly warns the demons causing Tokimitsu’s illness that, if they do not cure him, they will suffer in the great Avīchi hell.

The letter is traditionally called The Proof of the Lotus Sutra because it points out that all the Buddhas gave credence to the truth of the Lotus Sutra. However, it is also known as Prayer for a Return to Life from Fatal Illness because Tokimitsu was then battling a serious illness.

https://www.nichirenlibrary.org/en/wnd-1/Content/165