It is from a writing called, On Repaying Debts of Gratitude (Ho-on sho 報恩抄)
This is one of Nichiren's major writings. He wrote it to his fellows at Seichoji, the temple where he first started studying as a boy on the occasion of their teacher's passing.
This is a fuller quote of the passage:
Question: Is there a correct teaching that was not propagated even by T’ien-t’ai and Dengyō?
Answer: Yes, there is.
Question: What sort of teaching is it?
Answer: It consists of three things. It was left behind by the Buddha for the sake of those who live in the Latter Day of the Law. It is the correct teaching that was never propagated by Mahākāshyapa or Ānanda, Ashvaghosha or Nāgārjuna, T’ien-t’ai or Dengyō.
Question: What form does it take?
Answer: First, Japan and all the other countries throughout Jambudvīpa should all make the Shakyamuni Buddha of the essential teaching their object of devotion. In other words, the Shakyamuni and Many Treasures who appear in the treasure tower, all the other Buddhas, and the four bodhisattvas, including Superior Practices, will act as attendants to this Buddha. Second, there is the sanctuary of the essential teaching. Third, in Japan, China, India, and all the other countries of Jambudvīpa, every person, regardless of whether wise or ignorant, will set aside other practices and join in the chanting of Namu-myoho-renge-kyo. This teaching has never been taught before. Here in the entire land of Jambudvīpa, in all the 2,225 years since the passing of the Buddha, not a single person chanted it. Nichiren alone, without sparing his voice, now chants Namu-myoho-renge-kyo, Namu-myoho-renge-kyo.
The size of the waves depends upon the wind that raises them, the height of the flames depends upon how much firewood is piled on, the size of the lotuses depends upon the pond in which they grow, and the volume of rain depends upon the dragons that make it fall. The deeper the roots, the more prolific the branches. The farther the source, the longer the stream.
The Chou dynasty lasted for seven hundred years because of the propriety and filial devotion of its founder, King Wen. The Ch’in dynasty (221–206 b.c.e.), on the other hand, lasted hardly any time at all, because of the perverse ways of its founder, the First Emperor of the Ch’in. If Nichiren’s compassion is truly great and encompassing, Namu-myoho-renge-kyo will spread for ten thousand years and more, for all eternity, for it has the beneficial power to open the blind eyes of every living being in the country of Japan, and it blocks off the road that leads to the hell of incessant suffering. Its benefit surpasses that of Dengyō and T’ien-t’ai, and is superior to that of Nāgārjuna and Mahākāshyapa.
A hundred years of practice in the Land of Perfect Bliss cannot compare to the benefit gained from one day’s practice in the impure world. Two thousand years of propagating Buddhism during the Former and Middle Days of the Law are inferior to an hour of propagation in the Latter Day of the Law. This is in no way because of Nichiren’s wisdom, but simply because the time makes it so. In spring the blossoms open, in autumn the fruit appears. Summer is hot, winter is cold. The season makes it so, does it not?
“After I [the Buddha] have passed into extinction, in the last five-hundred-year period you must spread it abroad widely throughout Jambudvīpa and never allow it to be cut off, nor must you allow evil devils, the devils’ people, heavenly beings, dragons, yakshas, or kumbhānda demons to seize the advantage!”
If this passage of the Lotus Sutra should prove to be in vain, then Shāriputra will never become the Thus Come One Flower Glow, the Venerable Mahākāshyapa will never become the Thus Come One Light Bright, Maudgalyāyana will never become Tamalapattra Sandalwood Fragrance Buddha, Ānanda will never become Mountain Sea Wisdom Unrestricted Power King Buddha, the nun Mahāprajāpatī will never become the Buddha Gladly Seen by All Living Beings, and the nun Yashodharā will never become the Buddha Endowed with a Thousand Ten Thousand Glowing Marks. Major world system dust particle kalpas will then likewise be childish theory, and numberless major world system dust particle kalpas also will become a lie. Very likely Shakyamuni Buddha, the lord of teachings, will fall into the hell of incessant suffering, Many Treasures Buddha will be gasping amid the flames of the Avīchi hell, the Buddhas of the ten directions will have their home in the eight great hells, and all the various bodhisattvas will be forced to suffer in the 136 hells.
But how could such a thing ever be? Since the sutra’s prediction was not made in vain, then it is certain that all the people of Japan will chant Namu-myoho-renge-kyo!
Thus the flower will return to the root and the essence of the plant will remain in the earth. The benefit that I have been speaking of will surely accumulate in the life of the late Dōzen-bō. Namu-myoho-renge-kyo, Namu-myoho-renge-kyo.
The overall context of this statement is that it is now the Degenerate Age of the Dharma when the provisional teachings of Shakyamuni Buddha, the teachings taught in accord with the minds of others, have lost their efficacy. This is because those of us born in this Degenerate Age have not accumulated the karma necessary to benefit from those teachings. Instead, because we don't have the karma for them, they have the effect of harming us. This is, in a nutshell, the dilemma of Mappo that concerned the Buddhist community in 13ht c. Japan.
Honen's answer was to rely on Amitabha's vow and seek rebirth in Sukhavati. This answer was not acceptable to many because it was premised on the spiritual futility of this world. Nichiren fell in this camp. This world still held profound promise despite the start of the Degenerate Age. Notwithstanding, the problem of people's karmic capacity could not be avoided.
Nichiren found inspiration in the long standing view, particularly in the Tiantai/Tendai school that the Buddha's True teaching, the Ekayana, would spread in the Degenerate Age. In the Degenerate Age, life would be very difficult; Buddhist practice would be very difficult.
However, precisely because of this difficulty, practice in the Degenerate Age would be incredibly meritorious. That's the import of that quote.
This underlying view is from the Lotus Sutra, particularly in the Essential Section, ie. from the point that Prabhutaratna appears through the end of the sutra. I believe this view that this world being so difficult makes it an ideal place to practice is a general Mahayana view.