Day 17 covers all of Chapter 12, Devadatta, and opens Chapter 13, Encouragement for Keeping this Sutra.

Having last month covered the big reveal that Devadatta was the Buddha’s teacher, we come to the promise of future Buddhahood for Devadatta.

[Sakyamuni] said to the four kinds of devotees:

Devadatta will become a Buddha after innumerable kalpas. He will be called Heavenly-King, the Tathagata, the Deserver of Offerings, the Perfectly Enlightened One, the Man of Wisdom and Practice, the Well-Gone, the Knower of the World, the Unsurpassed Man, the Controller of Men, the Teacher of Gods and Men, the Buddha, the World-Honored One. The world of that Buddha will be called Heavenly-Way. That Buddha will live for twenty intermediate kalpas. He will expound the Wonderful Dharma to all living beings. [Hearing the Dharma from him,] as many living beings as there are sands in the River Ganges will obtain Arhatship; another group of innumerable living beings will aspire for the enlightenment of cause-knowers; and another group of living beings as many as there are sands in the River Ganges will aspire for unsurpassed enlightenment, obtain the truth of birthlessness, and reach the stage of irrevocability. After the Parinirvana of Heavenly-King Buddha, his right teachings will be preserved in that world for twenty intermediate kalpas. During that time a stupa of the seven treasures sixty yojanas tall and forty yojanas wide and deep will be erected to enshrine the sarrias of his perfect body. Gods and men will bow to the wonderful stupa of the seven treasures and offer various flowers, incense powder, incense to burn, incense applicable to the skin, garments, necklaces, banners, streamers, jeweled canopies, music and songs of praise [to the stupa]. [By doing all this,] innumerable living beings will attain Arhatship; another group of innumerable living beings will attain Pratyekabuddhahood; and another group of inconceivably numerous living beings will aspire for Bodhi and reach the stage of irrevocability.

The Daily Dharma of April 29, 2015, offers Nichiren’s explanation of the importance of Devadatta’s future Buddhahood.

This is indeed inexplicable yet precious. If Devadatta does not become a Buddha, the numerous evil people who were induced by him to enter into his evil comradeship would never be able to escape the torment of the Hell of Incessant Suffering. It is solely due to the great favor of the Lotus Sutra that all of Devadatta’s comrades, too, are allowed to be Buddhas.